a2p
accept
access
acct
addftinfo
addr2line
adjtime
afmtodit
after
aio_cancel
aio_error
aio_read
aio_return
aio_suspend
aio_waitcomplete
aio_write
alias
aliases
alloc
anvil
append
apply
apropos
ar
array
as
asa
asn1parse
at
atq
atrm
attemptckalloc
attemptckrealloc
authlib
authtest
autopoint
awk
b64decode
b64encode
basename
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bc
bdes
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biff
big5
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c89
c99
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checkbutton
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chio
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ckalloc
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clear
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clock
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close
cmp
co
col
colcrt
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comm
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compile_et
complete
compress
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connect
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continue
core
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cp
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creat
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cu
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cvs
date
dbiprof
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dc
dcgettext
dcngettext
dd
dde
default
defer
deliverquota
des
destroy
devfs
df
dgettext
dgst
dh
dhparam
dialog
diff
diff3
dig
dir
dirent
dirname
dirs
discard
disktab
dngettext
do
domainname
done
dprofpp
dsa
dsaparam
dtmfdecode
du
dup
dup2
eaccess
ec
ecdsa
echo
echotc
ecparam
ed
edit
editrc
ee
egrep
elf
elfdump
elif
else
enc
enc2xs
encoding
end
endif
endsw
engine
enigma
entry
env
envsubst
eof
eqn
err
errno
error
errstr
esac
ethers
euc
eui64
eval
event
evp
ex
exec
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expr
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f77
false
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fc
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fg
fgrep
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fhstat
fhstatfs
fi
file
file2c
fileevent
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flock
flush
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for
foreach
fork
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fs
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fsync
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g711conv
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i386_get_ioperm
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ident
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if
ifnames253
ifnames259
image
imapd
incr
indent
indxbib
info
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interp
intro
introduction
ioctl
ipcrm
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ipftest
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ippool
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jail_attach
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od
onintr
open
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perl
perl56delta
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FreeBSD/Linux/UNIX General Commands Manual
Hypertext Man Pages
perlfunc
 
PERLFUNC(1)	       Perl Programmers Reference Guide 	   PERLFUNC(1)



NAME
       perlfunc - Perl builtin functions

DESCRIPTION
       The functions in this section can serve as terms in an expression.
       They fall into two major categories: list operators and named unary
       operators.  These differ in their precedence relationship with a fol-
       lowing comma.  (See the precedence table in perlop.)  List operators
       take more than one argument, while unary operators can never take more
       than one argument.  Thus, a comma terminates the argument of a unary
       operator, but merely separates the arguments of a list operator.  A
       unary operator generally provides a scalar context to its argument,
       while a list operator may provide either scalar or list contexts for
       its arguments.  If it does both, the scalar arguments will be first,
       and the list argument will follow.  (Note that there can ever be only
       one such list argument.)  For instance, splice() has three scalar argu-
       ments followed by a list, whereas gethostbyname() has four scalar argu-
       ments.

       In the syntax descriptions that follow, list operators that expect a
       list (and provide list context for the elements of the list) are shown
       with LIST as an argument.  Such a list may consist of any combination
       of scalar arguments or list values; the list values will be included in
       the list as if each individual element were interpolated at that point
       in the list, forming a longer single-dimensional list value.  Commas
       should separate elements of the LIST.

       Any function in the list below may be used either with or without
       parentheses around its arguments.  (The syntax descriptions omit the
       parentheses.)  If you use the parentheses, the simple (but occasionally
       surprising) rule is this: It looks like a function, therefore it is a
       function, and precedence doesn't matter.  Otherwise it's a list opera-
       tor or unary operator, and precedence does matter.  And whitespace
       between the function and left parenthesis doesn't count--so you need to
       be careful sometimes:

	   print 1+2+4;        # Prints 7.
	   print(1+2) + 4;     # Prints 3.
	   print (1+2)+4;      # Also prints 3!
	   print +(1+2)+4;     # Prints 7.
	   print ((1+2)+4);    # Prints 7.

       If you run Perl with the -w switch it can warn you about this.  For
       example, the third line above produces:

	   print (...) interpreted as function at - line 1.
	   Useless use of integer addition in void context at - line 1.

       A few functions take no arguments at all, and therefore work as neither
       unary nor list operators.  These include such functions as "time" and
       "endpwent".  For example, "time+86_400" always means "time() + 86_400".

       For functions that can be used in either a scalar or list context, non-
       abortive failure is generally indicated in a scalar context by return-
       ing the undefined value, and in a list context by returning the null
       list.

       Remember the following important rule: There is no rule that relates
       the behavior of an expression in list context to its behavior in scalar
       context, or vice versa.	It might do two totally different things.
       Each operator and function decides which sort of value it would be most
       appropriate to return in scalar context.  Some operators return the
       length of the list that would have been returned in list context.  Some
       operators return the first value in the list.  Some operators return
       the last value in the list.  Some operators return a count of success-
       ful operations.	In general, they do what you want, unless you want
       consistency.

       A named array in scalar context is quite different from what would at
       first glance appear to be a list in scalar context.  You can't get a
       list like "(1,2,3)" into being in scalar context, because the compiler
       knows the context at compile time.  It would generate the scalar comma
       operator there, not the list construction version of the comma.	That
       means it was never a list to start with.

       In general, functions in Perl that serve as wrappers for system calls
       of the same name (like chown(2), fork(2), closedir(2), etc.) all return
       true when they succeed and "undef" otherwise, as is usually mentioned
       in the descriptions below.  This is different from the C interfaces,
       which return "-1" on failure.  Exceptions to this rule are "wait",
       "waitpid", and "syscall".  System calls also set the special $!	vari-
       able on failure.  Other functions do not, except accidentally.

       Perl Functions by Category

       Here are Perl's functions (including things that look like functions,
       like some keywords and named operators) arranged by category.  Some
       functions appear in more than one place.

       Functions for SCALARs or strings
	   "chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst",
	   "length", "oct", "ord", "pack", "q/STRING/", "qq/STRING/",
	   "reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst",
	   "y///"

       Regular expressions and pattern matching
	   "m//", "pos", "quotemeta", "s///", "split", "study", "qr//"

       Numeric functions
	   "abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand",
	   "sin", "sqrt", "srand"

       Functions for real @ARRAYs
	   "pop", "push", "shift", "splice", "unshift"

       Functions for list data
	   "grep", "join", "map", "qw/STRING/", "reverse", "sort", "unpack"

       Functions for real %HASHes
	   "delete", "each", "exists", "keys", "values"

       Input and output functions
	   "binmode", "close", "closedir", "dbmclose", "dbmopen", "die",
	   "eof", "fileno", "flock", "format", "getc", "print", "printf",
	   "read", "readdir", "rewinddir", "seek", "seekdir", "select",
	   "syscall", "sysread", "sysseek", "syswrite", "tell", "telldir",
	   "truncate", "warn", "write"

       Functions for fixed length data or records
	   "pack", "read", "syscall", "sysread", "syswrite", "unpack", "vec"

       Functions for filehandles, files, or directories
	   "-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob",
	   "ioctl", "link", "lstat", "mkdir", "open", "opendir", "readlink",
	   "rename", "rmdir", "stat", "symlink", "sysopen", "umask", "unlink",
	   "utime"

       Keywords related to the control flow of your Perl program
	   "caller", "continue", "die", "do", "dump", "eval", "exit", "goto",
	   "last", "next", "redo", "return", "sub", "wantarray"

       Keywords related to scoping
	   "caller", "import", "local", "my", "our", "package", "use"

       Miscellaneous functions
	   "defined", "dump", "eval", "formline", "local", "my", "our",
	   "reset", "scalar", "undef", "wantarray"

       Functions for processes and process groups
	   "alarm", "exec", "fork", "getpgrp", "getppid", "getpriority",
	   "kill", "pipe", "qx/STRING/", "setpgrp", "setpriority", "sleep",
	   "system", "times", "wait", "waitpid"

       Keywords related to perl modules
	   "do", "import", "no", "package", "require", "use"

       Keywords related to classes and object-orientedness
	   "bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied",
	   "untie", "use"

       Low-level socket functions
	   "accept", "bind", "connect", "getpeername", "getsockname", "get-
	   sockopt", "listen", "recv", "send", "setsockopt", "shutdown",
	   "socket", "socketpair"

       System V interprocess communication functions
	   "msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget",
	   "semop", "shmctl", "shmget", "shmread", "shmwrite"

       Fetching user and group info
	   "endgrent", "endhostent", "endnetent", "endpwent", "getgrent",
	   "getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam", "getp-
	   wuid", "setgrent", "setpwent"

       Fetching network info
	   "endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
	   "gethostent", "getnetbyaddr", "getnetbyname", "getnetent", "getpro-
	   tobyname", "getprotobynumber", "getprotoent", "getservbyname",
	   "getservbyport", "getservent", "sethostent", "setnetent", "setpro-
	   toent", "setservent"

       Time-related functions
	   "gmtime", "localtime", "time", "times"

       Functions new in perl5
	   "abs", "bless", "chomp", "chr", "exists", "formline", "glob",
	   "import", "lc", "lcfirst", "map", "my", "no", "our", "prototype",
	   "qx", "qw", "readline", "readpipe", "ref", "sub*", "sysopen",
	   "tie", "tied", "uc", "ucfirst", "untie", "use"

	   * - "sub" was a keyword in perl4, but in perl5 it is an operator,
	   which can be used in expressions.

       Functions obsoleted in perl5
	   "dbmclose", "dbmopen"

       Portability

       Perl was born in Unix and can therefore access all common Unix system
       calls.  In non-Unix environments, the functionality of some Unix system
       calls may not be available, or details of the available functionality
       may differ slightly.  The Perl functions affected by this are:

       "-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose",
       "dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endpro-
       toent", "endpwent", "endservent", "exec", "fcntl", "flock", "fork",
       "getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin",
       "getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp",
       "getpriority", "getprotobynumber", "getprotoent", "getpwent", "getpw-
       nam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob",
       "ioctl", "kill", "link", "lstat", "msgctl", "msgget", "msgrcv",
       "msgsnd", "open", "pipe", "readlink", "rename", "select", "semctl",
       "semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp",
       "setpriority", "setprotoent", "setpwent", "setservent", "setsockopt",
       "shmctl", "shmget", "shmread", "shmwrite", "socket", "socketpair",
       "stat", "symlink", "syscall", "sysopen", "system", "times", "truncate",
       "umask", "unlink", "utime", "wait", "waitpid"

       For more information about the portability of these functions, see
       perlport and other available platform-specific documentation.

       Alphabetical Listing of Perl Functions


       -X FILEHANDLE
       -X EXPR
       -X      A file test, where X is one of the letters listed below.  This
	       unary operator takes one argument, either a filename or a file-
	       handle, and tests the associated file to see if something is
	       true about it.  If the argument is omitted, tests $_, except
	       for "-t", which tests STDIN.  Unless otherwise documented, it
	       returns 1 for true and '' for false, or the undefined value if
	       the file doesn't exist.	Despite the funny names, precedence is
	       the same as any other named unary operator, and the argument
	       may be parenthesized like any other unary operator.  The opera-
	       tor may be any of:

		   -r  File is readable by effective uid/gid.
		   -w  File is writable by effective uid/gid.
		   -x  File is executable by effective uid/gid.
		   -o  File is owned by effective uid.

		   -R  File is readable by real uid/gid.
		   -W  File is writable by real uid/gid.
		   -X  File is executable by real uid/gid.
		   -O  File is owned by real uid.

		   -e  File exists.
		   -z  File has zero size (is empty).
		   -s  File has nonzero size (returns size in bytes).

		   -f  File is a plain file.
		   -d  File is a directory.
		   -l  File is a symbolic link.
		   -p  File is a named pipe (FIFO), or Filehandle is a pipe.
		   -S  File is a socket.
		   -b  File is a block special file.
		   -c  File is a character special file.
		   -t  Filehandle is opened to a tty.

		   -u  File has setuid bit set.
		   -g  File has setgid bit set.
		   -k  File has sticky bit set.

		   -T  File is an ASCII text file (heuristic guess).
		   -B  File is a "binary" file (opposite of -T).

		   -M  Script start time minus file modification time, in days.
		   -A  Same for access time.
		   -C  Same for inode change time (Unix, may differ for other platforms)

	       Example:

		   while (<>) {
		       chomp;
		       next unless -f $_;      # ignore specials
		       #...
		   }

	       The interpretation of the file permission operators "-r", "-R",
	       "-w", "-W", "-x", and "-X" is by default based solely on the
	       mode of the file and the uids and gids of the user.  There may
	       be other reasons you can't actually read, write, or execute the
	       file.  Such reasons may be for example network filesystem
	       access controls, ACLs (access control lists), read-only
	       filesystems, and unrecognized executable formats.

	       Also note that, for the superuser on the local filesystems, the
	       "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and
	       "-X" return 1 if any execute bit is set in the mode.  Scripts
	       run by the superuser may thus need to do a stat() to determine
	       the actual mode of the file, or temporarily set their effective
	       uid to something else.

	       If you are using ACLs, there is a pragma called "filetest" that
	       may produce more accurate results than the bare stat() mode
	       bits.  When under the "use filetest 'access'" the above-men-
	       tioned filetests will test whether the permission can (not) be
	       granted using the access() family of system calls.  Also note
	       that the "-x" and "-X" may under this pragma return true even
	       if there are no execute permission bits set (nor any extra exe-
	       cute permission ACLs).  This strangeness is due to the underly-
	       ing system calls' definitions.  Read the documentation for the
	       "filetest" pragma for more information.

	       Note that "-s/a/b/" does not do a negated substitution.	Saying
	       "-exp($foo)" still works as expected, however--only single let-
	       ters following a minus are interpreted as file tests.

	       The "-T" and "-B" switches work as follows.  The first block or
	       so of the file is examined for odd characters such as strange
	       control codes or characters with the high bit set.  If too many
	       strange characters (>30%) are found, it's a "-B" file; other-
	       wise it's a "-T" file.  Also, any file containing null in the
	       first block is considered a binary file.  If "-T" or "-B" is
	       used on a filehandle, the current IO buffer is examined rather
	       than the first block.  Both "-T" and "-B" return true on a null
	       file, or a file at EOF when testing a filehandle.  Because you
	       have to read a file to do the "-T" test, on most occasions you
	       want to use a "-f" against the file first, as in "next unless
	       -f $file && -T $file".

	       If any of the file tests (or either the "stat" or "lstat" oper-
	       ators) are given the special filehandle consisting of a soli-
	       tary underline, then the stat structure of the previous file
	       test (or stat operator) is used, saving a system call.  (This
	       doesn't work with "-t", and you need to remember that lstat()
	       and "-l" will leave values in the stat structure for the sym-
	       bolic link, not the real file.)	(Also, if the stat buffer was
	       filled by an "lstat" call, "-T" and "-B" will reset it with the
	       results of "stat _").  Example:

		   print "Can do.\n" if -r $a || -w _ || -x _;

		   stat($filename);
		   print "Readable\n" if -r _;
		   print "Writable\n" if -w _;
		   print "Executable\n" if -x _;
		   print "Setuid\n" if -u _;
		   print "Setgid\n" if -g _;
		   print "Sticky\n" if -k _;
		   print "Text\n" if -T _;
		   print "Binary\n" if -B _;

       abs VALUE
       abs     Returns the absolute value of its argument.  If VALUE is omit-
	       ted, uses $_.

       accept NEWSOCKET,GENERICSOCKET
	       Accepts an incoming socket connect, just as the accept(2) sys-
	       tem call does.  Returns the packed address if it succeeded,
	       false otherwise.  See the example in "Sockets: Client/Server
	       Communication" in perlipc.

	       On systems that support a close-on-exec flag on files, the flag
	       will be set for the newly opened file descriptor, as determined
	       by the value of $^F.  See "$^F" in perlvar.

       alarm SECONDS
       alarm   Arranges to have a SIGALRM delivered to this process after the
	       specified number of wallclock seconds has elapsed.  If SECONDS
	       is not specified, the value stored in $_ is used. (On some
	       machines, unfortunately, the elapsed time may be up to one sec-
	       ond less or more than you specified because of how seconds are
	       counted, and process scheduling may delay the delivery of the
	       signal even further.)

	       Only one timer may be counting at once.	Each call disables the
	       previous timer, and an argument of 0 may be supplied to cancel
	       the previous timer without starting a new one.  The returned
	       value is the amount of time remaining on the previous timer.

	       For delays of finer granularity than one second, you may use
	       Perl's four-argument version of select() leaving the first
	       three arguments undefined, or you might be able to use the
	       "syscall" interface to access setitimer(2) if your system sup-
	       ports it.  The Time::HiRes module (from CPAN, and starting from
	       Perl 5.8 part of the standard distribution) may also prove use-
	       ful.

	       It is usually a mistake to intermix "alarm" and "sleep" calls.
	       ("sleep" may be internally implemented in your system with
	       "alarm")

	       If you want to use "alarm" to time out a system call you need
	       to use an "eval"/"die" pair.  You can't rely on the alarm caus-
	       ing the system call to fail with $! set to "EINTR" because Perl
	       sets up signal handlers to restart system calls on some sys-
	       tems.  Using "eval"/"die" always works, modulo the caveats
	       given in "Signals" in perlipc.

		   eval {
		       local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
		       alarm $timeout;
		       $nread = sysread SOCKET, $buffer, $size;
		       alarm 0;
		   };
		   if ($@) {
		       die unless $@ eq "alarm\n";   # propagate unexpected errors
		       # timed out
		   }
		   else {
		       # didn't
		   }

	       For more information see perlipc.

       atan2 Y,X
	       Returns the arctangent of Y/X in the range -PI to PI.

	       For the tangent operation, you may use the "Math::Trig::tan"
	       function, or use the familiar relation:

		   sub tan { sin($_[0]) / cos($_[0])  }

	       Note that atan2(0, 0) is not well-defined.

       bind SOCKET,NAME
	       Binds a network address to a socket, just as the bind system
	       call does.  Returns true if it succeeded, false otherwise.
	       NAME should be a packed address of the appropriate type for the
	       socket.	See the examples in "Sockets: Client/Server Communica-
	       tion" in perlipc.

       binmode FILEHANDLE, LAYER
       binmode FILEHANDLE
	       Arranges for FILEHANDLE to be read or written in "binary" or
	       "text" mode on systems where the run-time libraries distinguish
	       between binary and text files.  If FILEHANDLE is an expression,
	       the value is taken as the name of the filehandle.  Returns true
	       on success, otherwise it returns "undef" and sets $! (errno).

	       On some systems (in general, DOS and Windows-based systems)
	       binmode() is necessary when you're not working with a text
	       file.  For the sake of portability it is a good idea to always
	       use it when appropriate, and to never use it when it isn't
	       appropriate.  Also, people can set their I/O to be by default
	       UTF-8 encoded Unicode, not bytes.

	       In other words: regardless of platform, use binmode() on binary
	       data, like for example images.

	       If LAYER is present it is a single string, but may contain mul-
	       tiple directives. The directives alter the behaviour of the
	       file handle.  When LAYER is present using binmode on text file
	       makes sense.

	       If LAYER is omitted or specified as ":raw" the filehandle is
	       made suitable for passing binary data. This includes turning
	       off possible CRLF translation and marking it as bytes (as
	       opposed to Unicode characters).	Note that, despite what may be
	       implied in "Programming Perl" (the Camel) or elsewhere, ":raw"
	       is not the simply inverse of ":crlf" -- other layers which
	       would affect binary nature of the stream are also disabled. See
	       PerlIO, perlrun and the discussion about the PERLIO environment
	       variable.

	       The ":bytes", ":crlf", and ":utf8", and any other directives of
	       the form ":...", are called I/O layers.	The "open" pragma can
	       be used to establish default I/O layers.  See open.

	       The LAYER parameter of the binmode() function is described as
	       "DISCIPLINE" in "Programming Perl, 3rd Edition".  However,
	       since the publishing of this book, by many known as "Camel
	       III", the consensus of the naming of this functionality has
	       moved from "discipline" to "layer".  All documentation of this
	       version of Perl therefore refers to "layers" rather than to
	       "disciplines".  Now back to the regularly scheduled documenta-
	       tion...

	       To mark FILEHANDLE as UTF-8, use ":utf8".

	       In general, binmode() should be called after open() but before
	       any I/O is done on the filehandle.  Calling binmode() will nor-
	       mally flush any pending buffered output data (and perhaps pend-
	       ing input data) on the handle.  An exception to this is the
	       ":encoding" layer that changes the default character encoding
	       of the handle, see open.  The ":encoding" layer sometimes needs
	       to be called in mid-stream, and it doesn't flush the stream.
	       The ":encoding" also implicitly pushes on top of itself the
	       ":utf8" layer because internally Perl will operate on UTF-8
	       encoded Unicode characters.

	       The operating system, device drivers, C libraries, and Perl
	       run-time system all work together to let the programmer treat a
	       single character ("\n") as the line terminator, irrespective of
	       the external representation.  On many operating systems, the
	       native text file representation matches the internal represen-
	       tation, but on some platforms the external representation of
	       "\n" is made up of more than one character.

	       Mac OS, all variants of Unix, and Stream_LF files on VMS use a
	       single character to end each line in the external representa-
	       tion of text (even though that single character is CARRIAGE
	       RETURN on Mac OS and LINE FEED on Unix and most VMS files). In
	       other systems like OS/2, DOS and the various flavors of MS-Win-
	       dows your program sees a "\n" as a simple "\cJ", but what's
	       stored in text files are the two characters "\cM\cJ".  That
	       means that, if you don't use binmode() on these systems,
	       "\cM\cJ" sequences on disk will be converted to "\n" on input,
	       and any "\n" in your program will be converted back to "\cM\cJ"
	       on output.  This is what you want for text files, but it can be
	       disastrous for binary files.

	       Another consequence of using binmode() (on some systems) is
	       that special end-of-file markers will be seen as part of the
	       data stream.  For systems from the Microsoft family this means
	       that if your binary data contains "\cZ", the I/O subsystem will
	       regard it as the end of the file, unless you use binmode().

	       binmode() is not only important for readline() and print()
	       operations, but also when using read(), seek(), sysread(),
	       syswrite() and tell() (see perlport for more details).  See the
	       $/ and "$\" variables in perlvar for how to manually set your
	       input and output line-termination sequences.

       bless REF,CLASSNAME
       bless REF
	       This function tells the thingy referenced by REF that it is now
	       an object in the CLASSNAME package.  If CLASSNAME is omitted,
	       the current package is used.  Because a "bless" is often the
	       last thing in a constructor, it returns the reference for con-
	       venience.  Always use the two-argument version if a derived
	       class might inherit the function doing the blessing.  See perl-
	       toot and perlobj for more about the blessing (and blessings) of
	       objects.

	       Consider always blessing objects in CLASSNAMEs that are mixed
	       case.  Namespaces with all lowercase names are considered
	       reserved for Perl pragmata.  Builtin types have all uppercase
	       names. To prevent confusion, you may wish to avoid such package
	       names as well.  Make sure that CLASSNAME is a true value.

	       See "Perl Modules" in perlmod.

       caller EXPR
       caller  Returns the context of the current subroutine call.  In scalar
	       context, returns the caller's package name if there is a
	       caller, that is, if we're in a subroutine or "eval" or
	       "require", and the undefined value otherwise.  In list context,
	       returns

		   ($package, $filename, $line) = caller;

	       With EXPR, it returns some extra information that the debugger
	       uses to print a stack trace.  The value of EXPR indicates how
	       many call frames to go back before the current one.

		   ($package, $filename, $line, $subroutine, $hasargs,
		   $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);

	       Here $subroutine may be "(eval)" if the frame is not a subrou-
	       tine call, but an "eval".  In such a case additional elements
	       $evaltext and $is_require are set: $is_require is true if the
	       frame is created by a "require" or "use" statement, $evaltext
	       contains the text of the "eval EXPR" statement.	In particular,
	       for an "eval BLOCK" statement, $filename is "(eval)", but
	       $evaltext is undefined.	(Note also that each "use" statement
	       creates a "require" frame inside an "eval EXPR" frame.)	$sub-
	       routine may also be "(unknown)" if this particular subroutine
	       happens to have been deleted from the symbol table.  $hasargs
	       is true if a new instance of @_ was set up for the frame.
	       $hints and $bitmask contain pragmatic hints that the caller was
	       compiled with.  The $hints and $bitmask values are subject to
	       change between versions of Perl, and are not meant for external
	       use.

	       Furthermore, when called from within the DB package, caller
	       returns more detailed information: it sets the list variable
	       @DB::args to be the arguments with which the subroutine was
	       invoked.

	       Be aware that the optimizer might have optimized call frames
	       away before "caller" had a chance to get the information.  That
	       means that caller(N) might not return information about the
	       call frame you expect it do, for "N > 1".  In particular,
	       @DB::args might have information from the previous time
	       "caller" was called.

       chdir EXPR
       chdir FILEHANDLE
       chdir DIRHANDLE
       chdir   Changes the working directory to EXPR, if possible. If EXPR is
	       omitted, changes to the directory specified by $ENV{HOME}, if
	       set; if not, changes to the directory specified by
	       $ENV{LOGDIR}. (Under VMS, the variable $ENV{SYS$LOGIN} is also
	       checked, and used if it is set.) If neither is set, "chdir"
	       does nothing. It returns true upon success, false otherwise.
	       See the example under "die".

	       On systems that support fchdir, you might pass a file handle or
	       directory handle as argument.  On systems that don't support
	       fchdir, passing handles produces a fatal error at run time.

       chmod LIST
	       Changes the permissions of a list of files.  The first element
	       of the list must be the numerical mode, which should probably
	       be an octal number, and which definitely should not be a string
	       of octal digits: 0644 is okay, '0644' is not.  Returns the num-
	       ber of files successfully changed.  See also "oct", if all you
	       have is a string.

		   $cnt = chmod 0755, 'foo', 'bar';
		   chmod 0755, @executables;
		   $mode = '0644'; chmod $mode, 'foo';	    # !!! sets mode to
							    # --w----r-T
		   $mode = '0644'; chmod oct($mode), 'foo'; # this is better
		   $mode = 0644;   chmod $mode, 'foo';	    # this is best

	       On systems that support fchmod, you might pass file handles
	       among the files.  On systems that don't support fchmod, passing
	       file handles produces a fatal error at run time.

		   open(my $fh, "<", "foo");
		   my $perm = (stat $fh)[2] & 07777;
		   chmod($perm | 0600, $fh);

	       You can also import the symbolic "S_I*" constants from the
	       Fcntl module:

		   use Fcntl ':mode';

		   chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
		   # This is identical to the chmod 0755 of the above example.

       chomp VARIABLE
       chomp( LIST )
       chomp   This safer version of "chop" removes any trailing string that
	       corresponds to the current value of $/ (also known as
	       $INPUT_RECORD_SEPARATOR in the "English" module).  It returns
	       the total number of characters removed from all its arguments.
	       It's often used to remove the newline from the end of an input
	       record when you're worried that the final record may be missing
	       its newline.  When in paragraph mode ("$/ = """), it removes
	       all trailing newlines from the string.  When in slurp mode ("$/
	       = undef") or fixed-length record mode ($/ is a reference to an
	       integer or the like, see perlvar) chomp() won't remove any-
	       thing.  If VARIABLE is omitted, it chomps $_.  Example:

		   while (<>) {
		       chomp;  # avoid \n on last field
		       @array = split(/:/);
		       # ...
		   }

	       If VARIABLE is a hash, it chomps the hash's values, but not its
	       keys.

	       You can actually chomp anything that's an lvalue, including an
	       assignment:

		   chomp($cwd = `pwd`);
		   chomp($answer = );

	       If you chomp a list, each element is chomped, and the total
	       number of characters removed is returned.

	       If the "encoding" pragma is in scope then the lengths returned
	       are calculated from the length of $/ in Unicode characters,
	       which is not always the same as the length of $/ in the native
	       encoding.

	       Note that parentheses are necessary when you're chomping any-
	       thing that is not a simple variable.  This is because "chomp
	       $cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;",
	       rather than as "chomp( $cwd = `pwd` )" which you might expect.
	       Similarly, "chomp $a, $b" is interpreted as "chomp($a), $b"
	       rather than as "chomp($a, $b)".

       chop VARIABLE
       chop( LIST )
       chop    Chops off the last character of a string and returns the char-
	       acter chopped.  It is much more efficient than "s/.$//s"
	       because it neither scans nor copies the string.	If VARIABLE is
	       omitted, chops $_.  If VARIABLE is a hash, it chops the hash's
	       values, but not its keys.

	       You can actually chop anything that's an lvalue, including an
	       assignment.

	       If you chop a list, each element is chopped.  Only the value of
	       the last "chop" is returned.

	       Note that "chop" returns the last character.  To return all but
	       the last character, use "substr($string, 0, -1)".

	       See also "chomp".

       chown LIST
	       Changes the owner (and group) of a list of files.  The first
	       two elements of the list must be the numeric uid and gid, in
	       that order.  A value of -1 in either position is interpreted by
	       most systems to leave that value unchanged.  Returns the number
	       of files successfully changed.

		   $cnt = chown $uid, $gid, 'foo', 'bar';
		   chown $uid, $gid, @filenames;

	       On systems that support fchown, you might pass file handles
	       among the files.  On systems that don't support fchown, passing
	       file handles produces a fatal error at run time.

	       Here's an example that looks up nonnumeric uids in the passwd
	       file:

		   print "User: ";
		   chomp($user = );
		   print "Files: ";
		   chomp($pattern = );

		   ($login,$pass,$uid,$gid) = getpwnam($user)
		       or die "$user not in passwd file";

		   @ary = glob($pattern);      # expand filenames
		   chown $uid, $gid, @ary;

	       On most systems, you are not allowed to change the ownership of
	       the file unless you're the superuser, although you should be
	       able to change the group to any of your secondary groups.  On
	       insecure systems, these restrictions may be relaxed, but this
	       is not a portable assumption.  On POSIX systems, you can detect
	       this condition this way:

		   use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
		   $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

       chr NUMBER
       chr     Returns the character represented by that NUMBER in the charac-
	       ter set.  For example, "chr(65)" is "A" in either ASCII or Uni-
	       code, and chr(0x263a) is a Unicode smiley face.	Note that
	       characters from 128 to 255 (inclusive) are by default not
	       encoded in UTF-8 Unicode for backward compatibility reasons
	       (but see encoding).

	       If NUMBER is omitted, uses $_.

	       For the reverse, use "ord".

	       Note that under the "bytes" pragma the NUMBER is masked to the
	       low eight bits.

	       See perlunicode and encoding for more about Unicode.

       chroot FILENAME
       chroot  This function works like the system call by the same name: it
	       makes the named directory the new root directory for all fur-
	       ther pathnames that begin with a "/" by your process and all
	       its children.  (It doesn't change your current working direc-
	       tory, which is unaffected.)  For security reasons, this call is
	       restricted to the superuser.  If FILENAME is omitted, does a
	       "chroot" to $_.

       close FILEHANDLE
       close   Closes the file or pipe associated with the file handle,
	       returning true only if IO buffers are successfully flushed and
	       closes the system file descriptor.  Closes the currently
	       selected filehandle if the argument is omitted.

	       You don't have to close FILEHANDLE if you are immediately going
	       to do another "open" on it, because "open" will close it for
	       you.  (See "open".)  However, an explicit "close" on an input
	       file resets the line counter ($.), while the implicit close
	       done by "open" does not.

	       If the file handle came from a piped open, "close" will addi-
	       tionally return false if one of the other system calls involved
	       fails, or if the program exits with non-zero status.  (If the
	       only problem was that the program exited non-zero, $! will be
	       set to 0.)  Closing a pipe also waits for the process executing
	       on the pipe to complete, in case you want to look at the output
	       of the pipe afterwards, and implicitly puts the exit status
	       value of that command into $?.

	       Prematurely closing the read end of a pipe (i.e. before the
	       process writing to it at the other end has closed it) will
	       result in a SIGPIPE being delivered to the writer.  If the
	       other end can't handle that, be sure to read all the data
	       before closing the pipe.

	       Example:

		   open(OUTPUT, '|sort >foo')  # pipe to sort
		       or die "Can't start sort: $!";
		   #... 		       # print stuff to output
		   close OUTPUT 	       # wait for sort to finish
		       or warn $! ? "Error closing sort pipe: $!"
				  : "Exit status $? from sort";
		   open(INPUT, 'foo')	       # get sort's results
		       or die "Can't open 'foo' for input: $!";

	       FILEHANDLE may be an expression whose value can be used as an
	       indirect filehandle, usually the real filehandle name.

       closedir DIRHANDLE
	       Closes a directory opened by "opendir" and returns the success
	       of that system call.

       connect SOCKET,NAME
	       Attempts to connect to a remote socket, just as the connect
	       system call does.  Returns true if it succeeded, false other-
	       wise.  NAME should be a packed address of the appropriate type
	       for the socket.	See the examples in "Sockets: Client/Server
	       Communication" in perlipc.

       continue BLOCK
	       "continue" is actually a flow control statement rather than a
	       function.  If there is a "continue" BLOCK attached to a BLOCK
	       (typically in a "while" or "foreach"), it is always executed
	       just before the conditional is about to be evaluated again,
	       just like the third part of a "for" loop in C.  Thus it can be
	       used to increment a loop variable, even when the loop has been
	       continued via the "next" statement (which is similar to the C
	       "continue" statement).

	       "last", "next", or "redo" may appear within a "continue" block.
	       "last" and "redo" will behave as if they had been executed
	       within the main block.  So will "next", but since it will exe-
	       cute a "continue" block, it may be more entertaining.

		   while (EXPR) {
		       ### redo always comes here
		       do_something;
		   } continue {
		       ### next always comes here
		       do_something_else;
		       # then back the top to re-check EXPR
		   }
		   ### last always comes here

	       Omitting the "continue" section is semantically equivalent to
	       using an empty one, logically enough.  In that case, "next"
	       goes directly back to check the condition at the top of the
	       loop.

       cos EXPR
       cos     Returns the cosine of EXPR (expressed in radians).  If EXPR is
	       omitted, takes cosine of $_.

	       For the inverse cosine operation, you may use the
	       "Math::Trig::acos()" function, or use this relation:

		   sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

       crypt PLAINTEXT,SALT
	       Creates a digest string exactly like the crypt(3) function in
	       the C library (assuming that you actually have a version there
	       that has not been extirpated as a potential munitions).

	       crypt() is a one-way hash function.  The PLAINTEXT and SALT is
	       turned into a short string, called a digest, which is returned.
	       The same PLAINTEXT and SALT will always return the same string,
	       but there is no (known) way to get the original PLAINTEXT from
	       the hash.  Small changes in the PLAINTEXT or SALT will result
	       in large changes in the digest.

	       There is no decrypt function.  This function isn't all that
	       useful for cryptography (for that, look for Crypt modules on
	       your nearby CPAN mirror) and the name "crypt" is a bit of a
	       misnomer.  Instead it is primarily used to check if two pieces
	       of text are the same without having to transmit or store the
	       text itself.  An example is checking if a correct password is
	       given.  The digest of the password is stored, not the password
	       itself.	The user types in a password that is crypt()'d with
	       the same salt as the stored digest.  If the two digests match
	       the password is correct.

	       When verifying an existing digest string you should use the
	       digest as the salt (like "crypt($plain, $digest) eq $digest").
	       The SALT used to create the digest is visible as part of the
	       digest.	This ensures crypt() will hash the new string with the
	       same salt as the digest.  This allows your code to work with
	       the standard crypt and with more exotic implementations.  In
	       other words, do not assume anything about the returned string
	       itself, or how many bytes in the digest matter.

	       Traditionally the result is a string of 13 bytes: two first
	       bytes of the salt, followed by 11 bytes from the set
	       "[./0-9A-Za-z]", and only the first eight bytes of the digest
	       string mattered, but alternative hashing schemes (like MD5),
	       higher level security schemes (like C2), and implementations on
	       non-UNIX platforms may produce different strings.

	       When choosing a new salt create a random two character string
	       whose characters come from the set "[./0-9A-Za-z]" (like "join
	       '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").
	       This set of characters is just a recommendation; the characters
	       allowed in the salt depend solely on your system's crypt
	       library, and Perl can't restrict what salts "crypt()" accepts.

	       Here's an example that makes sure that whoever runs this pro-
	       gram knows their password:

		   $pwd = (getpwuid($<))[1];

		   system "stty -echo";
		   print "Password: ";
		   chomp($word = );
		   print "\n";
		   system "stty echo";

		   if (crypt($word, $pwd) ne $pwd) {
		       die "Sorry...\n";
		   } else {
		       print "ok\n";
		   }

	       Of course, typing in your own password to whoever asks you for
	       it is unwise.

	       The crypt function is unsuitable for hashing large quantities
	       of data, not least of all because you can't get the information
	       back.  Look at the Digest module for more robust algorithms.

	       If using crypt() on a Unicode string (which potentially has
	       characters with codepoints above 255), Perl tries to make sense
	       of the situation by trying to downgrade (a copy of the string)
	       the string back to an eight-bit byte string before calling
	       crypt() (on that copy).	If that works, good.  If not, crypt()
	       dies with "Wide character in crypt".

       dbmclose HASH
	       [This function has been largely superseded by the "untie" func-
	       tion.]

	       Breaks the binding between a DBM file and a hash.

       dbmopen HASH,DBNAME,MASK
	       [This function has been largely superseded by the "tie" func-
	       tion.]

	       This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB
	       file to a hash.	HASH is the name of the hash.  (Unlike normal
	       "open", the first argument is not a filehandle, even though it
	       looks like one).  DBNAME is the name of the database (without
	       the .dir or .pag extension if any).  If the database does not
	       exist, it is created with protection specified by MASK (as mod-
	       ified by the "umask").  If your system supports only the older
	       DBM functions, you may perform only one "dbmopen" in your pro-
	       gram.  In older versions of Perl, if your system had neither
	       DBM nor ndbm, calling "dbmopen" produced a fatal error; it now
	       falls back to sdbm(3).

	       If you don't have write access to the DBM file, you can only
	       read hash variables, not set them.  If you want to test whether
	       you can write, either use file tests or try setting a dummy
	       hash entry inside an "eval", which will trap the error.

	       Note that functions such as "keys" and "values" may return huge
	       lists when used on large DBM files.  You may prefer to use the
	       "each" function to iterate over large DBM files.  Example:

		   # print out history file offsets
		   dbmopen(%HIST,'/usr/lib/news/history',0666);
		   while (($key,$val) = each %HIST) {
		       print $key, ' = ', unpack('L',$val), "\n";
		   }
		   dbmclose(%HIST);

	       See also AnyDBM_File for a more general description of the pros
	       and cons of the various dbm approaches, as well as DB_File for
	       a particularly rich implementation.

	       You can control which DBM library you use by loading that
	       library before you call dbmopen():

		   use DB_File;
		   dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
		       or die "Can't open netscape history file: $!";

       defined EXPR
       defined Returns a Boolean value telling whether EXPR has a value other
	       than the undefined value "undef".  If EXPR is not present, $_
	       will be checked.

	       Many operations return "undef" to indicate failure, end of
	       file, system error, uninitialized variable, and other excep-
	       tional conditions.  This function allows you to distinguish
	       "undef" from other values.  (A simple Boolean test will not
	       distinguish among "undef", zero, the empty string, and "0",
	       which are all equally false.)  Note that since "undef" is a
	       valid scalar, its presence doesn't necessarily indicate an
	       exceptional condition: "pop" returns "undef" when its argument
	       is an empty array, or when the element to return happens to be
	       "undef".

	       You may also use "defined(&func)" to check whether subroutine
	       &func has ever been defined.  The return value is unaffected by
	       any forward declarations of &func.  Note that a subroutine
	       which is not defined may still be callable: its package may
	       have an "AUTOLOAD" method that makes it spring into existence
	       the first time that it is called -- see perlsub.

	       Use of "defined" on aggregates (hashes and arrays) is depre-
	       cated.  It used to report whether memory for that aggregate has
	       ever been allocated.  This behavior may disappear in future
	       versions of Perl.  You should instead use a simple test for
	       size:

		   if (@an_array) { print "has array elements\n" }
		   if (%a_hash)   { print "has hash members\n"	 }

	       When used on a hash element, it tells you whether the value is
	       defined, not whether the key exists in the hash.  Use "exists"
	       for the latter purpose.

	       Examples:

		   print if defined $switch{'D'};
		   print "$val\n" while defined($val = pop(@ary));
		   die "Can't readlink $sym: $!"
		       unless defined($value = readlink $sym);
		   sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
		   $debugging = 0 unless defined $debugging;

	       Note:  Many folks tend to overuse "defined", and then are sur-
	       prised to discover that the number 0 and "" (the zero-length
	       string) are, in fact, defined values.  For example, if you say

		   "ab" =~ /a(.*)b/;

	       The pattern match succeeds, and $1 is defined, despite the fact
	       that it matched "nothing".  It didn't really fail to match any-
	       thing.  Rather, it matched something that happened to be zero
	       characters long.  This is all very above-board and honest.
	       When a function returns an undefined value, it's an admission
	       that it couldn't give you an honest answer.  So you should use
	       "defined" only when you're questioning the integrity of what
	       you're trying to do.  At other times, a simple comparison to 0
	       or "" is what you want.

	       See also "undef", "exists", "ref".

       delete EXPR
	       Given an expression that specifies a hash element, array ele-
	       ment, hash slice, or array slice, deletes the specified ele-
	       ment(s) from the hash or array.	In the case of an array, if
	       the array elements happen to be at the end, the size of the
	       array will shrink to the highest element that tests true for
	       exists() (or 0 if no such element exists).

	       Returns a list with the same number of elements as the number
	       of elements for which deletion was attempted.  Each element of
	       that list consists of either the value of the element deleted,
	       or the undefined value.	In scalar context, this means that you
	       get the value of the last element deleted (or the undefined
	       value if that element did not exist).

		   %hash = (foo => 11, bar => 22, baz => 33);
		   $scalar = delete $hash{foo}; 	    # $scalar is 11
		   $scalar = delete @hash{qw(foo bar)};     # $scalar is 22
		   @array  = delete @hash{qw(foo bar baz)}; # @array  is (undef,undef,33)

	       Deleting from %ENV modifies the environment.  Deleting from a
	       hash tied to a DBM file deletes the entry from the DBM file.
	       Deleting from a "tie"d hash or array may not necessarily return
	       anything.

	       Deleting an array element effectively returns that position of
	       the array to its initial, uninitialized state.  Subsequently
	       testing for the same element with exists() will return false.
	       Also, deleting array elements in the middle of an array will
	       not shift the index of the elements after them down.  Use
	       splice() for that.  See "exists".

	       The following (inefficiently) deletes all the values of %HASH
	       and @ARRAY:

		   foreach $key (keys %HASH) {
		       delete $HASH{$key};
		   }

		   foreach $index (0 .. $#ARRAY) {
		       delete $ARRAY[$index];
		   }

	       And so do these:

		   delete @HASH{keys %HASH};

		   delete @ARRAY[0 .. $#ARRAY];

	       But both of these are slower than just assigning the empty list
	       or undefining %HASH or @ARRAY:

		   %HASH = ();	       # completely empty %HASH
		   undef %HASH;        # forget %HASH ever existed

		   @ARRAY = ();        # completely empty @ARRAY
		   undef @ARRAY;       # forget @ARRAY ever existed

	       Note that the EXPR can be arbitrarily complicated as long as
	       the final operation is a hash element, array element,  hash
	       slice, or array slice lookup:

		   delete $ref->[$x][$y]{$key};
		   delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

		   delete $ref->[$x][$y][$index];
		   delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];

       die LIST
	       Outside an "eval", prints the value of LIST to "STDERR" and
	       exits with the current value of $! (errno).  If $! is 0, exits
	       with the value of "($? >> 8)" (backtick `command` status).  If
	       "($? >> 8)" is 0, exits with 255.  Inside an "eval()," the
	       error message is stuffed into $@ and the "eval" is terminated
	       with the undefined value.  This makes "die" the way to raise an
	       exception.

	       Equivalent examples:

		   die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
		   chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

	       If the last element of LIST does not end in a newline, the cur-
	       rent script line number and input line number (if any) are also
	       printed, and a newline is supplied.  Note that the "input line
	       number" (also known as "chunk") is subject to whatever notion
	       of "line" happens to be currently in effect, and is also avail-
	       able as the special variable $..  See "$/" in perlvar and "$."
	       in perlvar.

	       Hint: sometimes appending ", stopped" to your message will
	       cause it to make better sense when the string "at foo line 123"
	       is appended.  Suppose you are running script "canasta".

		   die "/etc/games is no good";
		   die "/etc/games is no good, stopped";

	       produce, respectively

		   /etc/games is no good at canasta line 123.
		   /etc/games is no good, stopped at canasta line 123.

	       See also exit(), warn(), and the Carp module.

	       If LIST is empty and $@ already contains a value (typically
	       from a previous eval) that value is reused after appending
	       "\t...propagated".  This is useful for propagating exceptions:

		   eval { ... };
		   die unless $@ =~ /Expected exception/;

	       If LIST is empty and $@ contains an object reference that has a
	       "PROPAGATE" method, that method will be called with additional
	       file and line number parameters.  The return value replaces the
	       value in $@.  i.e. as if "$@ = eval { $@->PROPAGATE(__FILE__,
	       __LINE__) };" were called.

	       If $@ is empty then the string "Died" is used.

	       die() can also be called with a reference argument.  If this
	       happens to be trapped within an eval(), $@ contains the refer-
	       ence.  This behavior permits a more elaborate exception han-
	       dling implementation using objects that maintain arbitrary
	       state about the nature of the exception.  Such a scheme is
	       sometimes preferable to matching particular string values of $@
	       using regular expressions.  Here's an example:

		   use Scalar::Util 'blessed';

		   eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
		   if ($@) {
		       if (blessed($@) && $@->isa("Some::Module::Exception")) {
			   # handle Some::Module::Exception
		       }
		       else {
			   # handle all other possible exceptions
		       }
		   }

	       Because perl will stringify uncaught exception messages before
	       displaying them, you may want to overload stringification oper-
	       ations on such custom exception objects.  See overload for
	       details about that.

	       You can arrange for a callback to be run just before the "die"
	       does its deed, by setting the $SIG{__DIE__} hook.  The associ-
	       ated handler will be called with the error text and can change
	       the error message, if it sees fit, by calling "die" again.  See
	       "$SIG{expr}" in perlvar for details on setting %SIG entries,
	       and "eval BLOCK" for some examples.  Although this feature was
	       to be run only right before your program was to exit, this is
	       not currently the case--the $SIG{__DIE__} hook is currently
	       called even inside eval()ed blocks/strings!  If one wants the
	       hook to do nothing in such situations, put

		       die @_ if $^S;

	       as the first line of the handler (see "$^S" in perlvar).
	       Because this promotes strange action at a distance, this coun-
	       terintuitive behavior may be fixed in a future release.

       do BLOCK
	       Not really a function.  Returns the value of the last command
	       in the sequence of commands indicated by BLOCK.	When modified
	       by the "while" or "until" loop modifier, executes the BLOCK
	       once before testing the loop condition. (On other statements
	       the loop modifiers test the conditional first.)

	       "do BLOCK" does not count as a loop, so the loop control state-
	       ments "next", "last", or "redo" cannot be used to leave or
	       restart the block.  See perlsyn for alternative strategies.

       do SUBROUTINE(LIST)
	       This form of subroutine call is deprecated.  See perlsub.

       do EXPR Uses the value of EXPR as a filename and executes the contents
	       of the file as a Perl script.

		   do 'stat.pl';

	       is just like

		   eval `cat stat.pl`;

	       except that it's more efficient and concise, keeps track of the
	       current filename for error messages, searches the @INC directo-
	       ries, and updates %INC if the file is found.  See "Predefined
	       Names" in perlvar for these variables.  It also differs in that
	       code evaluated with "do FILENAME" cannot see lexicals in the
	       enclosing scope; "eval STRING" does.  It's the same, however,
	       in that it does reparse the file every time you call it, so you
	       probably don't want to do this inside a loop.

	       If "do" cannot read the file, it returns undef and sets $! to
	       the error.  If "do" can read the file but cannot compile it, it
	       returns undef and sets an error message in $@.	If the file is
	       successfully compiled, "do" returns the value of the last
	       expression evaluated.

	       Note that inclusion of library modules is better done with the
	       "use" and "require" operators, which also do automatic error
	       checking and raise an exception if there's a problem.

	       You might like to use "do" to read in a program configuration
	       file.  Manual error checking can be done this way:

		   # read in config files: system first, then user
		   for $file ("/share/prog/defaults.rc",
			      "$ENV{HOME}/.someprogrc")
		  {
		       unless ($return = do $file) {
			   warn "couldn't parse $file: $@" if $@;
			   warn "couldn't do $file: $!"    unless defined $return;
			   warn "couldn't run $file"	   unless $return;
		       }
		   }

       dump LABEL
       dump    This function causes an immediate core dump.  See also the -u
	       command-line switch in perlrun, which does the same thing.
	       Primarily this is so that you can use the undump program (not
	       supplied) to turn your core dump into an executable binary
	       after having initialized all your variables at the beginning of
	       the program.  When the new binary is executed it will begin by
	       executing a "goto LABEL" (with all the restrictions that "goto"
	       suffers).  Think of it as a goto with an intervening core dump
	       and reincarnation.  If "LABEL" is omitted, restarts the program
	       from the top.

	       WARNING: Any files opened at the time of the dump will not be
	       open any more when the program is reincarnated, with possible
	       resulting confusion on the part of Perl.

	       This function is now largely obsolete, partly because it's very
	       hard to convert a core file into an executable, and because the
	       real compiler backends for generating portable bytecode and
	       compilable C code have superseded it.  That's why you should
	       now invoke it as "CORE::dump()", if you don't want to be warned
	       against a possible typo.

	       If you're looking to use dump to speed up your program, con-
	       sider generating bytecode or native C code as described in
	       perlcc.	If you're just trying to accelerate a CGI script, con-
	       sider using the "mod_perl" extension to Apache, or the CPAN
	       module, CGI::Fast.  You might also consider autoloading or
	       selfloading, which at least make your program appear to run
	       faster.

       each HASH
	       When called in list context, returns a 2-element list consist-
	       ing of the key and value for the next element of a hash, so
	       that you can iterate over it.  When called in scalar context,
	       returns only the key for the next element in the hash.

	       Entries are returned in an apparently random order.  The actual
	       random order is subject to change in future versions of perl,
	       but it is guaranteed to be in the same order as either the
	       "keys" or "values" function would produce on the same (unmodi-
	       fied) hash.  Since Perl 5.8.1 the ordering is different even
	       between different runs of Perl for security reasons (see "Algo-
	       rithmic Complexity Attacks" in perlsec).

	       When the hash is entirely read, a null array is returned in
	       list context (which when assigned produces a false (0) value),
	       and "undef" in scalar context.  The next call to "each" after
	       that will start iterating again.  There is a single iterator
	       for each hash, shared by all "each", "keys", and "values" func-
	       tion calls in the program; it can be reset by reading all the
	       elements from the hash, or by evaluating "keys HASH" or "values
	       HASH".  If you add or delete elements of a hash while you're
	       iterating over it, you may get entries skipped or duplicated,
	       so don't.  Exception: It is always safe to delete the item most
	       recently returned by "each()", which means that the following
	       code will work:

		       while (($key, $value) = each %hash) {
			 print $key, "\n";
			 delete $hash{$key};   # This is safe
		       }

	       The following prints out your environment like the printenv(1)
	       program, only in a different order:

		   while (($key,$value) = each %ENV) {
		       print "$key=$value\n";
		   }

	       See also "keys", "values" and "sort".

       eof FILEHANDLE
       eof ()
       eof     Returns 1 if the next read on FILEHANDLE will return end of
	       file, or if FILEHANDLE is not open.  FILEHANDLE may be an
	       expression whose value gives the real filehandle.  (Note that
	       this function actually reads a character and then "ungetc"s it,
	       so isn't very useful in an interactive context.)  Do not read
	       from a terminal file (or call "eof(FILEHANDLE)" on it) after
	       end-of-file is reached.	File types such as terminals may lose
	       the end-of-file condition if you do.

	       An "eof" without an argument uses the last file read.  Using
	       "eof()" with empty parentheses is very different.  It refers to
	       the pseudo file formed from the files listed on the command
	       line and accessed via the "<>" operator.  Since "<>" isn't
	       explicitly opened, as a normal filehandle is, an "eof()" before
	       "<>" has been used will cause @ARGV to be examined to determine
	       if input is available.	Similarly, an "eof()" after "<>" has
	       returned end-of-file will assume you are processing another
	       @ARGV list, and if you haven't set @ARGV, will read input from
	       "STDIN"; see "I/O Operators" in perlop.

	       In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to
	       detect the end of each file, "eof()" will only detect the end
	       of the last file.  Examples:

		   # reset line numbering on each input file
		   while (<>) {
		       next if /^\s*#/;        # skip comments
		       print "$.\t$_";
		   } continue {
		       close ARGV  if eof;     # Not eof()!
		   }

		   # insert dashes just before last line of last file
		   while (<>) {
		       if (eof()) {	       # check for end of last file
			   print "--------------\n";
		       }
		       print;
		       last if eof();	       # needed if we're reading from a terminal
		   }

	       Practical hint: you almost never need to use "eof" in Perl,
	       because the input operators typically return "undef" when they
	       run out of data, or if there was an error.

       eval EXPR
       eval BLOCK
       eval    In the first form, the return value of EXPR is parsed and exe-
	       cuted as if it were a little Perl program.  The value of the
	       expression (which is itself determined within scalar context)
	       is first parsed, and if there weren't any errors, executed in
	       the lexical context of the current Perl program, so that any
	       variable settings or subroutine and format definitions remain
	       afterwards.  Note that the value is parsed every time the
	       "eval" executes.  If EXPR is omitted, evaluates $_.  This form
	       is typically used to delay parsing and subsequent execution of
	       the text of EXPR until run time.

	       In the second form, the code within the BLOCK is parsed only
	       once--at the same time the code surrounding the "eval" itself
	       was parsed--and executed within the context of the current Perl
	       program.  This form is typically used to trap exceptions more
	       efficiently than the first (see below), while also providing
	       the benefit of checking the code within BLOCK at compile time.

	       The final semicolon, if any, may be omitted from the value of
	       EXPR or within the BLOCK.

	       In both forms, the value returned is the value of the last
	       expression evaluated inside the mini-program; a return state-
	       ment may be also used, just as with subroutines.  The expres-
	       sion providing the return value is evaluated in void, scalar,
	       or list context, depending on the context of the "eval" itself.
	       See "wantarray" for more on how the evaluation context can be
	       determined.

	       If there is a syntax error or runtime error, or a "die" state-
	       ment is executed, an undefined value is returned by "eval", and
	       $@ is set to the error message.	If there was no error, $@ is
	       guaranteed to be a null string.	Beware that using "eval" nei-
	       ther silences perl from printing warnings to STDERR, nor does
	       it stuff the text of warning messages into $@.  To do either of
	       those, you have to use the $SIG{__WARN__} facility, or turn off
	       warnings inside the BLOCK or EXPR using "no warnings 'all'".
	       See "warn", perlvar, warnings and perllexwarn.

	       Note that, because "eval" traps otherwise-fatal errors, it is
	       useful for determining whether a particular feature (such as
	       "socket" or "symlink") is implemented.  It is also Perl's
	       exception trapping mechanism, where the die operator is used to
	       raise exceptions.

	       If the code to be executed doesn't vary, you may use the eval-
	       BLOCK form to trap run-time errors without incurring the
	       penalty of recompiling each time.  The error, if any, is still
	       returned in $@.	Examples:

		   # make divide-by-zero nonfatal
		   eval { $answer = $a / $b; }; warn $@ if $@;

		   # same thing, but less efficient
		   eval '$answer = $a / $b'; warn $@ if $@;

		   # a compile-time error
		   eval { $answer = };		       # WRONG

		   # a run-time error
		   eval '$answer =';   # sets $@

	       Using the "eval{}" form as an exception trap in libraries does
	       have some issues.  Due to the current arguably broken state of
	       "__DIE__" hooks, you may wish not to trigger any "__DIE__"
	       hooks that user code may have installed.  You can use the
	       "local $SIG{__DIE__}" construct for this purpose, as shown in
	       this example:

		   # a very private exception trap for divide-by-zero
		   eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
		   warn $@ if $@;

	       This is especially significant, given that "__DIE__" hooks can
	       call "die" again, which has the effect of changing their error
	       messages:

		   # __DIE__ hooks may modify error messages
		   {
		      local $SIG{'__DIE__'} =
			     sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
		      eval { die "foo lives here" };
		      print $@ if $@;		     # prints "bar lives here"
		   }

	       Because this promotes action at a distance, this counterintu-
	       itive behavior may be fixed in a future release.

	       With an "eval", you should be especially careful to remember
	       what's being looked at when:

		   eval $x;	       # CASE 1
		   eval "$x";	       # CASE 2

		   eval '$x';	       # CASE 3
		   eval { $x };        # CASE 4

		   eval "\$$x++";      # CASE 5
		   $$x++;	       # CASE 6

	       Cases 1 and 2 above behave identically: they run the code con-
	       tained in the variable $x.  (Although case 2 has misleading
	       double quotes making the reader wonder what else might be hap-
	       pening (nothing is).)  Cases 3 and 4 likewise behave in the
	       same way: they run the code '$x', which does nothing but return
	       the value of $x.  (Case 4 is preferred for purely visual rea-
	       sons, but it also has the advantage of compiling at compile-
	       time instead of at run-time.)  Case 5 is a place where normally
	       you would like to use double quotes, except that in this par-
	       ticular situation, you can just use symbolic references
	       instead, as in case 6.

	       "eval BLOCK" does not count as a loop, so the loop control
	       statements "next", "last", or "redo" cannot be used to leave or
	       restart the block.

	       Note that as a very special case, an "eval ''" executed within
	       the "DB" package doesn't see the usual surrounding lexical
	       scope, but rather the scope of the first non-DB piece of code
	       that called it. You don't normally need to worry about this
	       unless you are writing a Perl debugger.

       exec LIST
       exec PROGRAM LIST
	       The "exec" function executes a system command and never
	       returns-- use "system" instead of "exec" if you want it to
	       return.	It fails and returns false only if the command does
	       not exist and it is executed directly instead of via your sys-
	       tem's command shell (see below).

	       Since it's a common mistake to use "exec" instead of "system",
	       Perl warns you if there is a following statement which isn't
	       "die", "warn", or "exit" (if "-w" is set  -  but you always do
	       that).	If you really want to follow an "exec" with some other
	       statement, you can use one of these styles to avoid the warn-
	       ing:

		   exec ('foo')   or print STDERR "couldn't exec foo: $!";
		   { exec ('foo') }; print STDERR "couldn't exec foo: $!";

	       If there is more than one argument in LIST, or if LIST is an
	       array with more than one value, calls execvp(3) with the argu-
	       ments in LIST.  If there is only one scalar argument or an
	       array with one element in it, the argument is checked for shell
	       metacharacters, and if there are any, the entire argument is
	       passed to the system's command shell for parsing (this is
	       "/bin/sh -c" on Unix platforms, but varies on other platforms).
	       If there are no shell metacharacters in the argument, it is
	       split into words and passed directly to "execvp", which is more
	       efficient.  Examples:

		   exec '/bin/echo', 'Your arguments are: ', @ARGV;
		   exec "sort $outfile | uniq";

	       If you don't really want to execute the first argument, but
	       want to lie to the program you are executing about its own
	       name, you can specify the program you actually want to run as
	       an "indirect object" (without a comma) in front of the LIST.
	       (This always forces interpretation of the LIST as a multivalued
	       list, even if there is only a single scalar in the list.)
	       Example:

		   $shell = '/bin/csh';
		   exec $shell '-sh';	       # pretend it's a login shell

	       or, more directly,

		   exec {'/bin/csh'} '-sh';    # pretend it's a login shell

	       When the arguments get executed via the system shell, results
	       will be subject to its quirks and capabilities.	See "`STRING`"
	       in perlop for details.

	       Using an indirect object with "exec" or "system" is also more
	       secure.	This usage (which also works fine with system())
	       forces interpretation of the arguments as a multivalued list,
	       even if the list had just one argument.	That way you're safe
	       from the shell expanding wildcards or splitting up words with
	       whitespace in them.

		   @args = ( "echo surprise" );

		   exec @args;		     # subject to shell escapes
					       # if @args == 1
		   exec { $args[0] } @args;  # safe even with one-arg list

	       The first version, the one without the indirect object, ran the
	       echo program, passing it "surprise" an argument.  The second
	       version didn't--it tried to run a program literally called
	       "echo surprise", didn't find it, and set $? to a non-zero value
	       indicating failure.

	       Beginning with v5.6.0, Perl will attempt to flush all files
	       opened for output before the exec, but this may not be sup-
	       ported on some platforms (see perlport).  To be safe, you may
	       need to set $| ($AUTOFLUSH in English) or call the "aut-
	       oflush()" method of "IO::Handle" on any open handles in order
	       to avoid lost output.

	       Note that "exec" will not call your "END" blocks, nor will it
	       call any "DESTROY" methods in your objects.

       exists EXPR
	       Given an expression that specifies a hash element or array ele-
	       ment, returns true if the specified element in the hash or
	       array has ever been initialized, even if the corresponding
	       value is undefined.  The element is not autovivified if it
	       doesn't exist.

		   print "Exists\n"    if exists $hash{$key};
		   print "Defined\n"   if defined $hash{$key};
		   print "True\n"      if $hash{$key};

		   print "Exists\n"    if exists $array[$index];
		   print "Defined\n"   if defined $array[$index];
		   print "True\n"      if $array[$index];

	       A hash or array element can be true only if it's defined, and
	       defined if it exists, but the reverse doesn't necessarily hold
	       true.

	       Given an expression that specifies the name of a subroutine,
	       returns true if the specified subroutine has ever been
	       declared, even if it is undefined.  Mentioning a subroutine
	       name for exists or defined does not count as declaring it.
	       Note that a subroutine which does not exist may still be
	       callable: its package may have an "AUTOLOAD" method that makes
	       it spring into existence the first time that it is called --
	       see perlsub.

		   print "Exists\n"    if exists &subroutine;
		   print "Defined\n"   if defined &subroutine;

	       Note that the EXPR can be arbitrarily complicated as long as
	       the final operation is a hash or array key lookup or subroutine
	       name:

		   if (exists $ref->{A}->{B}->{$key})  { }
		   if (exists $hash{A}{B}{$key})       { }

		   if (exists $ref->{A}->{B}->[$ix])   { }
		   if (exists $hash{A}{B}[$ix])        { }

		   if (exists &{$ref->{A}{B}{$key}})   { }

	       Although the deepest nested array or hash will not spring into
	       existence just because its existence was tested, any interven-
	       ing ones will.  Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}"
	       will spring into existence due to the existence test for the
	       $key element above.  This happens anywhere the arrow operator
	       is used, including even:

		   undef $ref;
		   if (exists $ref->{"Some key"})      { }
		   print $ref;		   # prints HASH(0x80d3d5c)

	       This surprising autovivification in what does not at first--or
	       even second--glance appear to be an lvalue context may be fixed
	       in a future release.

	       See "Pseudo-hashes: Using an array as a hash" in perlref for
	       specifics on how exists() acts when used on a pseudo-hash.

	       Use of a subroutine call, rather than a subroutine name, as an
	       argument to exists() is an error.

		   exists ⊂        # OK
		   exists &sub();      # Error

       exit EXPR
       exit    Evaluates EXPR and exits immediately with that value.	Exam-
	       ple:

		   $ans = ;
		   exit 0 if $ans =~ /^[Xx]/;

	       See also "die".	If EXPR is omitted, exits with 0 status.  The
	       only universally recognized values for EXPR are 0 for success
	       and 1 for error; other values are subject to interpretation
	       depending on the environment in which the Perl program is run-
	       ning.  For example, exiting 69 (EX_UNAVAILABLE) from a sendmail
	       incoming-mail filter will cause the mailer to return the item
	       undelivered, but that's not true everywhere.

	       Don't use "exit" to abort a subroutine if there's any chance
	       that someone might want to trap whatever error happened.  Use
	       "die" instead, which can be trapped by an "eval".

	       The exit() function does not always exit immediately.  It calls
	       any defined "END" routines first, but these "END" routines may
	       not themselves abort the exit.  Likewise any object destructors
	       that need to be called are called before the real exit.	If
	       this is a problem, you can call "POSIX:_exit($status)" to avoid
	       END and destructor processing.  See perlmod for details.

       exp EXPR
       exp     Returns e (the natural logarithm base) to the power of EXPR.
	       If EXPR is omitted, gives "exp($_)".

       fcntl FILEHANDLE,FUNCTION,SCALAR
	       Implements the fcntl(2) function.  You'll probably have to say

		   use Fcntl;

	       first to get the correct constant definitions.  Argument pro-
	       cessing and value return works just like "ioctl" below.	For
	       example:

		   use Fcntl;
		   fcntl($filehandle, F_GETFL, $packed_return_buffer)
		       or die "can't fcntl F_GETFL: $!";

	       You don't have to check for "defined" on the return from
	       "fcntl".  Like "ioctl", it maps a 0 return from the system call
	       into "0 but true" in Perl.  This string is true in boolean con-
	       text and 0 in numeric context.  It is also exempt from the nor-
	       mal -w warnings on improper numeric conversions.

	       Note that "fcntl" will produce a fatal error if used on a
	       machine that doesn't implement fcntl(2).  See the Fcntl module
	       or your fcntl(2) manpage to learn what functions are available
	       on your system.

	       Here's an example of setting a filehandle named "REMOTE" to be
	       non-blocking at the system level.  You'll have to negotiate $|
	       on your own, though.

		   use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

		   $flags = fcntl(REMOTE, F_GETFL, 0)
			       or die "Can't get flags for the socket: $!\n";

		   $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
			       or die "Can't set flags for the socket: $!\n";

       fileno FILEHANDLE
	       Returns the file descriptor for a filehandle, or undefined if
	       the filehandle is not open.  This is mainly useful for con-
	       structing bitmaps for "select" and low-level POSIX tty-handling
	       operations.  If FILEHANDLE is an expression, the value is taken
	       as an indirect filehandle, generally its name.

	       You can use this to find out whether two handles refer to the
	       same underlying descriptor:

		   if (fileno(THIS) == fileno(THAT)) {
		       print "THIS and THAT are dups\n";
		   }

	       (Filehandles connected to memory objects via new features of
	       "open" may return undefined even though they are open.)

       flock FILEHANDLE,OPERATION
	       Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns
	       true for success, false on failure.  Produces a fatal error if
	       used on a machine that doesn't implement flock(2), fcntl(2)
	       locking, or lockf(3).  "flock" is Perl's portable file locking
	       interface, although it locks only entire files, not records.

	       Two potentially non-obvious but traditional "flock" semantics
	       are that it waits indefinitely until the lock is granted, and
	       that its locks merely advisory.	Such discretionary locks are
	       more flexible, but offer fewer guarantees.  This means that
	       programs that do not also use "flock" may modify files locked
	       with "flock".  See perlport, your port's specific documenta-
	       tion, or your system-specific local manpages for details.  It's
	       best to assume traditional behavior if you're writing portable
	       programs.  (But if you're not, you should as always feel per-
	       fectly free to write for your own system's idiosyncrasies
	       (sometimes called "features").  Slavish adherence to portabil-
	       ity concerns shouldn't get in the way of your getting your job
	       done.)

	       OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly com-
	       bined with LOCK_NB.  These constants are traditionally valued
	       1, 2, 8 and 4, but you can use the symbolic names if you import
	       them from the Fcntl module, either individually, or as a group
	       using the ':flock' tag.	LOCK_SH requests a shared lock,
	       LOCK_EX requests an exclusive lock, and LOCK_UN releases a pre-
	       viously requested lock.	If LOCK_NB is bitwise-or'ed with
	       LOCK_SH or LOCK_EX then "flock" will return immediately rather
	       than blocking waiting for the lock (check the return status to
	       see if you got it).

	       To avoid the possibility of miscoordination, Perl now flushes
	       FILEHANDLE before locking or unlocking it.

	       Note that the emulation built with lockf(3) doesn't provide
	       shared locks, and it requires that FILEHANDLE be open with
	       write intent.  These are the semantics that lockf(3) imple-
	       ments.  Most if not all systems implement lockf(3) in terms of
	       fcntl(2) locking, though, so the differing semantics shouldn't
	       bite too many people.

	       Note that the fcntl(2) emulation of flock(3) requires that
	       FILEHANDLE be open with read intent to use LOCK_SH and requires
	       that it be open with write intent to use LOCK_EX.

	       Note also that some versions of "flock" cannot lock things over
	       the network; you would need to use the more system-specific
	       "fcntl" for that.  If you like you can force Perl to ignore
	       your system's flock(2) function, and so provide its own
	       fcntl(2)-based emulation, by passing the switch "-Ud_flock" to
	       the Configure program when you configure perl.

	       Here's a mailbox appender for BSD systems.

		   use Fcntl ':flock'; # import LOCK_* constants

		   sub lock {
		       flock(MBOX,LOCK_EX);
		       # and, in case someone appended
		       # while we were waiting...
		       seek(MBOX, 0, 2);
		   }

		   sub unlock {
		       flock(MBOX,LOCK_UN);
		   }

		   open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
			   or die "Can't open mailbox: $!";

		   lock();
		   print MBOX $msg,"\n\n";
		   unlock();

	       On systems that support a real flock(), locks are inherited
	       across fork() calls, whereas those that must resort to the more
	       capricious fcntl() function lose the locks, making it harder to
	       write servers.

	       See also DB_File for other flock() examples.

       fork    Does a fork(2) system call to create a new process running the
	       same program at the same point.	It returns the child pid to
	       the parent process, 0 to the child process, or "undef" if the
	       fork is unsuccessful.  File descriptors (and sometimes locks on
	       those descriptors) are shared, while everything else is copied.
	       On most systems supporting fork(), great care has gone into
	       making it extremely efficient (for example, using copy-on-write
	       technology on data pages), making it the dominant paradigm for
	       multitasking over the last few decades.

	       Beginning with v5.6.0, Perl will attempt to flush all files
	       opened for output before forking the child process, but this
	       may not be supported on some platforms (see perlport).  To be
	       safe, you may need to set $| ($AUTOFLUSH in English) or call
	       the "autoflush()" method of "IO::Handle" on any open handles in
	       order to avoid duplicate output.

	       If you "fork" without ever waiting on your children, you will
	       accumulate zombies.  On some systems, you can avoid this by
	       setting $SIG{CHLD} to "IGNORE".	See also perlipc for more
	       examples of forking and reaping moribund children.

	       Note that if your forked child inherits system file descriptors
	       like STDIN and STDOUT that are actually connected by a pipe or
	       socket, even if you exit, then the remote server (such as, say,
	       a CGI script or a backgrounded job launched from a remote
	       shell) won't think you're done.	You should reopen those to
	       /dev/null if it's any issue.

       format  Declare a picture format for use by the "write" function.  For
	       example:

		   format Something =
		       Test: @<<<<<<<< @||||| @>>>>>
			     $str,     $%,    '$' . int($num)
		   .

		   $str = "widget";
		   $num = $cost/$quantity;
		   $~ = 'Something';
		   write;

	       See perlform for many details and examples.

       formline PICTURE,LIST
	       This is an internal function used by "format"s, though you may
	       call it, too.  It formats (see perlform) a list of values
	       according to the contents of PICTURE, placing the output into
	       the format output accumulator, $^A (or $ACCUMULATOR in
	       English).  Eventually, when a "write" is done, the contents of
	       $^A are written to some filehandle.  You could also read $^A
	       and then set $^A back to "".  Note that a format typically does
	       one "formline" per line of form, but the "formline" function
	       itself doesn't care how many newlines are embedded in the PIC-
	       TURE.  This means that the "~" and "~~" tokens will treat the
	       entire PICTURE as a single line.  You may therefore need to use
	       multiple formlines to implement a single record format, just
	       like the format compiler.

	       Be careful if you put double quotes around the picture, because
	       an "@" character may be taken to mean the beginning of an array
	       name.  "formline" always returns true.  See perlform for other
	       examples.

       getc FILEHANDLE
       getc    Returns the next character from the input file attached to
	       FILEHANDLE, or the undefined value at end of file, or if there
	       was an error (in the latter case $! is set).  If FILEHANDLE is
	       omitted, reads from STDIN.  This is not particularly efficient.
	       However, it cannot be used by itself to fetch single characters
	       without waiting for the user to hit enter.  For that, try some-
	       thing more like:

		   if ($BSD_STYLE) {
		       system "stty cbreak /dev/tty 2>&1";
		   }
		   else {
		       system "stty", '-icanon', 'eol', "\001";
		   }

		   $key = getc(STDIN);

		   if ($BSD_STYLE) {
		       system "stty -cbreak /dev/tty 2>&1";
		   }
		   else {
		       system "stty", 'icanon', 'eol', '^@'; # ASCII null
		   }
		   print "\n";

	       Determination of whether $BSD_STYLE should be set is left as an
	       exercise to the reader.

	       The "POSIX::getattr" function can do this more portably on sys-
	       tems purporting POSIX compliance.  See also the "Term::ReadKey"
	       module from your nearest CPAN site; details on CPAN can be
	       found on "CPAN" in perlmodlib.

       getlogin
	       This implements the C library function of the same name, which
	       on most systems returns the current login from /etc/utmp, if
	       any.  If null, use "getpwuid".

		   $login = getlogin || getpwuid($<) || "Kilroy";

	       Do not consider "getlogin" for authentication: it is not as
	       secure as "getpwuid".

       getpeername SOCKET
	       Returns the packed sockaddr address of other end of the SOCKET
	       connection.

		   use Socket;
		   $hersockaddr    = getpeername(SOCK);
		   ($port, $iaddr) = sockaddr_in($hersockaddr);
		   $herhostname    = gethostbyaddr($iaddr, AF_INET);
		   $herstraddr	   = inet_ntoa($iaddr);

       getpgrp PID
	       Returns the current process group for the specified PID.  Use a
	       PID of 0 to get the current process group for the current
	       process.  Will raise an exception if used on a machine that
	       doesn't implement getpgrp(2).  If PID is omitted, returns
	       process group of current process.  Note that the POSIX version
	       of "getpgrp" does not accept a PID argument, so only "PID==0"
	       is truly portable.

       getppid Returns the process id of the parent process.

	       Note for Linux users: on Linux, the C functions "getpid()" and
	       "getppid()" return different values from different threads. In
	       order to be portable, this behavior is not reflected by the
	       perl-level function "getppid()", that returns a consistent
	       value across threads. If you want to call the underlying "getp-
	       pid()", you may use the CPAN module "Linux::Pid".

       getpriority WHICH,WHO
	       Returns the current priority for a process, a process group, or
	       a user.	(See getpriority(2).)  Will raise a fatal exception if
	       used on a machine that doesn't implement getpriority(2).

       getpwnam NAME
       getgrnam NAME
       gethostbyname NAME
       getnetbyname NAME
       getprotobyname NAME
       getpwuid UID
       getgrgid GID
       getservbyname NAME,PROTO
       gethostbyaddr ADDR,ADDRTYPE
       getnetbyaddr ADDR,ADDRTYPE
       getprotobynumber NUMBER
       getservbyport PORT,PROTO
       getpwent
       getgrent
       gethostent
       getnetent
       getprotoent
       getservent
       setpwent
       setgrent
       sethostent STAYOPEN
       setnetent STAYOPEN
       setprotoent STAYOPEN
       setservent STAYOPEN
       endpwent
       endgrent
       endhostent
       endnetent
       endprotoent
       endservent
	       These routines perform the same functions as their counterparts
	       in the system library.  In list context, the return values from
	       the various get routines are as follows:

		   ($name,$passwd,$uid,$gid,
		      $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
		   ($name,$passwd,$gid,$members) = getgr*
		   ($name,$aliases,$addrtype,$length,@addrs) = gethost*
		   ($name,$aliases,$addrtype,$net) = getnet*
		   ($name,$aliases,$proto) = getproto*
		   ($name,$aliases,$port,$proto) = getserv*

	       (If the entry doesn't exist you get a null list.)

	       The exact meaning of the $gcos field varies but it usually con-
	       tains the real name of the user (as opposed to the login name)
	       and other information pertaining to the user.  Beware, however,
	       that in many system users are able to change this information
	       and therefore it cannot be trusted and therefore the $gcos is
	       tainted (see perlsec).  The $passwd and $shell, user's
	       encrypted password and login shell, are also tainted, because
	       of the same reason.

	       In scalar context, you get the name, unless the function was a
	       lookup by name, in which case you get the other thing, whatever
	       it is.  (If the entry doesn't exist you get the undefined
	       value.)	For example:

		   $uid   = getpwnam($name);
		   $name  = getpwuid($num);
		   $name  = getpwent();
		   $gid   = getgrnam($name);
		   $name  = getgrgid($num);
		   $name  = getgrent();
		   #etc.

	       In getpw*() the fields $quota, $comment, and $expire are spe-
	       cial cases in the sense that in many systems they are unsup-
	       ported.	If the $quota is unsupported, it is an empty scalar.
	       If it is supported, it usually encodes the disk quota.  If the
	       $comment field is unsupported, it is an empty scalar.  If it is
	       supported it usually encodes some administrative comment about
	       the user.  In some systems the $quota field may be $change or
	       $age, fields that have to do with password aging.  In some sys-
	       tems the $comment field may be $class.  The $expire field, if
	       present, encodes the expiration period of the account or the
	       password.  For the availability and the exact meaning of these
	       fields in your system, please consult your getpwnam(3) documen-
	       tation and your pwd.h file.  You can also find out from within
	       Perl what your $quota and $comment fields mean and whether you
	       have the $expire field by using the "Config" module and the
	       values "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and
	       "d_pwexpire".  Shadow password files are only supported if your
	       vendor has implemented them in the intuitive fashion that call-
	       ing the regular C library routines gets the shadow versions if
	       you're running under privilege or if there exists the shadow(3)
	       functions as found in System V (this includes Solaris and
	       Linux.)	Those systems that implement a proprietary shadow
	       password facility are unlikely to be supported.

	       The $members value returned by getgr*() is a space separated
	       list of the login names of the members of the group.

	       For the gethost*() functions, if the "h_errno" variable is sup-
	       ported in C, it will be returned to you via $? if the function
	       call fails.  The @addrs value returned by a successful call is
	       a list of the raw addresses returned by the corresponding sys-
	       tem library call.  In the Internet domain, each address is four
	       bytes long and you can unpack it by saying something like:

		   ($a,$b,$c,$d) = unpack('C4',$addr[0]);

	       The Socket library makes this slightly easier:

		   use Socket;
		   $iaddr = inet_aton("127.1"); # or whatever address
		   $name  = gethostbyaddr($iaddr, AF_INET);

		   # or going the other way
		   $straddr = inet_ntoa($iaddr);

	       If you get tired of remembering which element of the return
	       list contains which return value, by-name interfaces are pro-
	       vided in standard modules: "File::stat", "Net::hostent",
	       "Net::netent", "Net::protoent", "Net::servent", "Time::gmtime",
	       "Time::localtime", and "User::grent".  These override the nor-
	       mal built-ins, supplying versions that return objects with the
	       appropriate names for each field.  For example:

		  use File::stat;
		  use User::pwent;
		  $is_his = (stat($filename)->uid == pwent($whoever)->uid);

	       Even though it looks like they're the same method calls (uid),
	       they aren't, because a "File::stat" object is different from a
	       "User::pwent" object.

       getsockname SOCKET
	       Returns the packed sockaddr address of this end of the SOCKET
	       connection, in case you don't know the address because you have
	       several different IPs that the connection might have come in
	       on.

		   use Socket;
		   $mysockaddr = getsockname(SOCK);
		   ($port, $myaddr) = sockaddr_in($mysockaddr);
		   printf "Connect to %s [%s]\n",
		      scalar gethostbyaddr($myaddr, AF_INET),
		      inet_ntoa($myaddr);

       getsockopt SOCKET,LEVEL,OPTNAME
	       Queries the option named OPTNAME associated with SOCKET at a
	       given LEVEL.  Options may exist at multiple protocol levels
	       depending on the socket type, but at least the uppermost socket
	       level SOL_SOCKET (defined in the "Socket" module) will exist.
	       To query options at another level the protocol number of the
	       appropriate protocol controlling the option should be supplied.
	       For example, to indicate that an option is to be interpreted by
	       the TCP protocol, LEVEL should be set to the protocol number of
	       TCP, which you can get using getprotobyname.

	       The call returns a packed string representing the requested
	       socket option, or "undef" if there is an error (the error rea-
	       son will be in $!). What exactly is in the packed string
	       depends in the LEVEL and OPTNAME, consult your system documen-
	       tation for details. A very common case however is that the
	       option is an integer, in which case the result will be a packed
	       integer which you can decode using unpack with the "i" (or "I")
	       format.

	       An example testing if Nagle's algorithm is turned on on a
	       socket:

		   use Socket qw(:all);

		   defined(my $tcp = getprotobyname("tcp"))
		       or die "Could not determine the protocol number for tcp";
		   # my $tcp = IPPROTO_TCP; # Alternative
		   my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
		       or die "Could not query TCP_NODELAY socket option: $!";
		   my $nodelay = unpack("I", $packed);
		   print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";

       glob EXPR
       glob    In list context, returns a (possibly empty) list of filename
	       expansions on the value of EXPR such as the standard Unix shell
	       /bin/csh would do. In scalar context, glob iterates through
	       such filename expansions, returning undef when the list is
	       exhausted. This is the internal function implementing the
	       "<*.c>" operator, but you can use it directly. If EXPR is omit-
	       ted, $_ is used.  The "<*.c>" operator is discussed in more
	       detail in "I/O Operators" in perlop.

	       Beginning with v5.6.0, this operator is implemented using the
	       standard "File::Glob" extension.  See File::Glob for details.

       gmtime EXPR
       gmtime  Converts a time as returned by the time function to an 9-ele-
	       ment list with the time localized for the standard Greenwich
	       time zone.  Typically used as follows:

		   #  0    1	2     3     4	 5     6     7	   8
		   ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
							   gmtime(time);

	       All list elements are numeric, and come straight out of the C
	       `struct tm'.  $sec, $min, and $hour are the seconds, minutes,
	       and hours of the specified time.  $mday is the day of the
	       month, and $mon is the month itself, in the range 0..11 with 0
	       indicating January and 11 indicating December.  $year is the
	       number of years since 1900.  That is, $year is 123 in year
	       2023.  $wday is the day of the week, with 0 indicating Sunday
	       and 3 indicating Wednesday.  $yday is the day of the year, in
	       the range 0..364 (or 0..365 in leap years).  $isdst is always
	       0.

	       Note that the $year element is not simply the last two digits
	       of the year.  If you assume it is then you create non-Y2K-com-
	       pliant programs--and you wouldn't want to do that, would you?

	       The proper way to get a complete 4-digit year is simply:

		       $year += 1900;

	       And to get the last two digits of the year (e.g., '01' in 2001)
	       do:

		       $year = sprintf("%02d", $year % 100);

	       If EXPR is omitted, "gmtime()" uses the current time
	       ("gmtime(time)").

	       In scalar context, "gmtime()" returns the ctime(3) value:

		   $now_string = gmtime;  # e.g., "Thu Oct 13 04:54:34 1994"

	       If you need local time instead of GMT use the "localtime"
	       builtin.  See also the "timegm" function provided by the
	       "Time::Local" module, and the strftime(3) and mktime(3) func-
	       tions available via the POSIX module.

	       This scalar value is not locale dependent (see perllocale), but
	       is instead a Perl builtin.  To get somewhat similar but locale
	       dependent date strings, see the example in "localtime".

	       See "gmtime" in perlport for portability concerns.

       goto LABEL
       goto EXPR
       goto &NAME
	       The "goto-LABEL" form finds the statement labeled with LABEL
	       and resumes execution there.  It may not be used to go into any
	       construct that requires initialization, such as a subroutine or
	       a "foreach" loop.  It also can't be used to go into a construct
	       that is optimized away, or to get out of a block or subroutine
	       given to "sort".  It can be used to go almost anywhere else
	       within the dynamic scope, including out of subroutines, but
	       it's usually better to use some other construct such as "last"
	       or "die".  The author of Perl has never felt the need to use
	       this form of "goto" (in Perl, that is--C is another matter).
	       (The difference being that C does not offer named loops com-
	       bined with loop control.  Perl does, and this replaces most
	       structured uses of "goto" in other languages.)

	       The "goto-EXPR" form expects a label name, whose scope will be
	       resolved dynamically.  This allows for computed "goto"s per
	       FORTRAN, but isn't necessarily recommended if you're optimizing
	       for maintainability:

		   goto ("FOO", "BAR", "GLARCH")[$i];

	       The "goto-&NAME" form is quite different from the other forms
	       of "goto".  In fact, it isn't a goto in the normal sense at
	       all, and doesn't have the stigma associated with other gotos.
	       Instead, it exits the current subroutine (losing any changes
	       set by local()) and immediately calls in its place the named
	       subroutine using the current value of @_.  This is used by
	       "AUTOLOAD" subroutines that wish to load another subroutine and
	       then pretend that the other subroutine had been called in the
	       first place (except that any modifications to @_ in the current
	       subroutine are propagated to the other subroutine.)  After the
	       "goto", not even "caller" will be able to tell that this rou-
	       tine was called first.

	       NAME needn't be the name of a subroutine; it can be a scalar
	       variable containing a code reference, or a block that evaluates
	       to a code reference.

       grep BLOCK LIST
       grep EXPR,LIST
	       This is similar in spirit to, but not the same as, grep(1) and
	       its relatives.  In particular, it is not limited to using regu-
	       lar expressions.

	       Evaluates the BLOCK or EXPR for each element of LIST (locally
	       setting $_ to each element) and returns the list value consist-
	       ing of those elements for which the expression evaluated to
	       true.  In scalar context, returns the number of times the
	       expression was true.

		   @foo = grep(!/^#/, @bar);	# weed out comments

	       or equivalently,

		   @foo = grep {!/^#/} @bar;	# weed out comments

	       Note that $_ is an alias to the list value, so it can be used
	       to modify the elements of the LIST.  While this is useful and
	       supported, it can cause bizarre results if the elements of LIST
	       are not variables.  Similarly, grep returns aliases into the
	       original list, much as a for loop's index variable aliases the
	       list elements.  That is, modifying an element of a list
	       returned by grep (for example, in a "foreach", "map" or another
	       "grep") actually modifies the element in the original list.
	       This is usually something to be avoided when writing clear
	       code.

	       See also "map" for a list composed of the results of the BLOCK
	       or EXPR.

       hex EXPR
       hex     Interprets EXPR as a hex string and returns the corresponding
	       value.  (To convert strings that might start with either 0,
	       "0x", or "0b", see "oct".)  If EXPR is omitted, uses $_.

		   print hex '0xAf'; # prints '175'
		   print hex 'aF';   # same

	       Hex strings may only represent integers.  Strings that would
	       cause integer overflow trigger a warning.  Leading whitespace
	       is not stripped, unlike oct(). To present something as hex,
	       look into "printf", "sprintf", or "unpack".

       import LIST
	       There is no builtin "import" function.  It is just an ordinary
	       method (subroutine) defined (or inherited) by modules that wish
	       to export names to another module.  The "use" function calls
	       the "import" method for the package used.  See also "use",
	       perlmod, and Exporter.

       index STR,SUBSTR,POSITION
       index STR,SUBSTR
	       The index function searches for one string within another, but
	       without the wildcard-like behavior of a full regular-expression
	       pattern match.  It returns the position of the first occurrence
	       of SUBSTR in STR at or after POSITION.  If POSITION is omitted,
	       starts searching from the beginning of the string.  POSITION
	       before the beginning of the string or after its end is treated
	       as if it were the beginning or the end, respectively.  POSITION
	       and the return value are based at 0 (or whatever you've set the
	       $[ variable to--but don't do that).  If the substring is not
	       found, "index" returns one less than the base, ordinarily "-1".

       int EXPR
       int     Returns the integer portion of EXPR.  If EXPR is omitted, uses
	       $_.  You should not use this function for rounding: one because
	       it truncates towards 0, and two because machine representations
	       of floating point numbers can sometimes produce counterintu-
	       itive results.  For example, "int(-6.725/0.025)" produces -268
	       rather than the correct -269; that's because it's really more
	       like -268.99999999999994315658 instead.	Usually, the
	       "sprintf", "printf", or the "POSIX::floor" and "POSIX::ceil"
	       functions will serve you better than will int().

       ioctl FILEHANDLE,FUNCTION,SCALAR
	       Implements the ioctl(2) function.  You'll probably first have
	       to say

		   require "sys/ioctl.ph";     # probably in $Config{archlib}/sys/ioctl.ph

	       to get the correct function definitions.  If sys/ioctl.ph
	       doesn't exist or doesn't have the correct definitions you'll
	       have to roll your own, based on your C header files such as
	       .  (There is a Perl script called h2ph that comes
	       with the Perl kit that may help you in this, but it's nontriv-
	       ial.)  SCALAR will be read and/or written depending on the
	       FUNCTION--a pointer to the string value of SCALAR will be
	       passed as the third argument of the actual "ioctl" call.  (If
	       SCALAR has no string value but does have a numeric value, that
	       value will be passed rather than a pointer to the string value.
	       To guarantee this to be true, add a 0 to the scalar before
	       using it.)  The "pack" and "unpack" functions may be needed to
	       manipulate the values of structures used by "ioctl".

	       The return value of "ioctl" (and "fcntl") is as follows:

		       if OS returns:	       then Perl returns:
			   -1			 undefined value
			    0		       string "0 but true"
		       anything else		   that number

	       Thus Perl returns true on success and false on failure, yet you
	       can still easily determine the actual value returned by the
	       operating system:

		   $retval = ioctl(...) || -1;
		   printf "System returned %d\n", $retval;

	       The special string "0 but true" is exempt from -w complaints
	       about improper numeric conversions.

       join EXPR,LIST
	       Joins the separate strings of LIST into a single string with
	       fields separated by the value of EXPR, and returns that new
	       string.	Example:

		   $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

	       Beware that unlike "split", "join" doesn't take a pattern as
	       its first argument.  Compare "split".

       keys HASH
	       Returns a list consisting of all the keys of the named hash.
	       (In scalar context, returns the number of keys.)

	       The keys are returned in an apparently random order.  The
	       actual random order is subject to change in future versions of
	       perl, but it is guaranteed to be the same order as either the
	       "values" or "each" function produces (given that the hash has
	       not been modified).  Since Perl 5.8.1 the ordering is different
	       even between different runs of Perl for security reasons (see
	       "Algorithmic Complexity Attacks" in perlsec).

	       As a side effect, calling keys() resets the HASH's internal
	       iterator (see "each").  In particular, calling keys() in void
	       context resets the iterator with no other overhead.

	       Here is yet another way to print your environment:

		   @keys = keys %ENV;
		   @values = values %ENV;
		   while (@keys) {
		       print pop(@keys), '=', pop(@values), "\n";
		   }

	       or how about sorted by key:

		   foreach $key (sort(keys %ENV)) {
		       print $key, '=', $ENV{$key}, "\n";
		   }

	       The returned values are copies of the original keys in the
	       hash, so modifying them will not affect the original hash.
	       Compare "values".

	       To sort a hash by value, you'll need to use a "sort" function.
	       Here's a descending numeric sort of a hash by its values:

		   foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
		       printf "%4d %s\n", $hash{$key}, $key;
		   }

	       As an lvalue "keys" allows you to increase the number of hash
	       buckets allocated for the given hash.  This can gain you a mea-
	       sure of efficiency if you know the hash is going to get big.
	       (This is similar to pre-extending an array by assigning a
	       larger number to $#array.)  If you say

		   keys %hash = 200;

	       then %hash will have at least 200 buckets allocated for it--256
	       of them, in fact, since it rounds up to the next power of two.
	       These buckets will be retained even if you do "%hash = ()", use
	       "undef %hash" if you want to free the storage while %hash is
	       still in scope.	You can't shrink the number of buckets allo-
	       cated for the hash using "keys" in this way (but you needn't
	       worry about doing this by accident, as trying has no effect).

	       See also "each", "values" and "sort".

       kill SIGNAL, LIST
	       Sends a signal to a list of processes.  Returns the number of
	       processes successfully signaled (which is not necessarily the
	       same as the number actually killed).

		   $cnt = kill 1, $child1, $child2;
		   kill 9, @goners;

	       If SIGNAL is zero, no signal is sent to the process.  This is a
	       useful way to check that a child process is alive and hasn't
	       changed its UID.  See perlport for notes on the portability of
	       this construct.

	       Unlike in the shell, if SIGNAL is negative, it kills process
	       groups instead of processes.  (On System V, a negative PROCESS
	       number will also kill process groups, but that's not portable.)
	       That means you usually want to use positive not negative sig-
	       nals.  You may also use a signal name in quotes.

	       See "Signals" in perlipc for more details.

       last LABEL
       last    The "last" command is like the "break" statement in C (as used
	       in loops); it immediately exits the loop in question.  If the
	       LABEL is omitted, the command refers to the innermost enclosing
	       loop.  The "continue" block, if any, is not executed:

		   LINE: while () {
		       last LINE if /^$/;      # exit when done with header
		       #...
		   }

	       "last" cannot be used to exit a block which returns a value
	       such as "eval {}", "sub {}" or "do {}", and should not be used
	       to exit a grep() or map() operation.

	       Note that a block by itself is semantically identical to a loop
	       that executes once.  Thus "last" can be used to effect an early
	       exit out of such a block.

	       See also "continue" for an illustration of how "last", "next",
	       and "redo" work.

       lc EXPR
       lc      Returns a lowercased version of EXPR.  This is the internal
	       function implementing the "\L" escape in double-quoted strings.
	       Respects current LC_CTYPE locale if "use locale" in force.  See
	       perllocale and perlunicode for more details about locale and
	       Unicode support.

	       If EXPR is omitted, uses $_.

       lcfirst EXPR
       lcfirst Returns the value of EXPR with the first character lowercased.
	       This is the internal function implementing the "\l" escape in
	       double-quoted strings.  Respects current LC_CTYPE locale if
	       "use locale" in force.  See perllocale and perlunicode for more
	       details about locale and Unicode support.

	       If EXPR is omitted, uses $_.

       length EXPR
       length  Returns the length in characters of the value of EXPR.  If EXPR
	       is omitted, returns length of $_.  Note that this cannot be
	       used on an entire array or hash to find out how many elements
	       these have.  For that, use "scalar @array" and "scalar keys
	       %hash" respectively.

	       Note the characters: if the EXPR is in Unicode, you will get
	       the number of characters, not the number of bytes.  To get the
	       length in bytes, use "do { use bytes; length(EXPR) }", see
	       bytes.

       link OLDFILE,NEWFILE
	       Creates a new filename linked to the old filename.  Returns
	       true for success, false otherwise.

       listen SOCKET,QUEUESIZE
	       Does the same thing that the listen system call does.  Returns
	       true if it succeeded, false otherwise.  See the example in
	       "Sockets: Client/Server Communication" in perlipc.

       local EXPR
	       You really probably want to be using "my" instead, because
	       "local" isn't what most people think of as "local".  See "Pri-
	       vate Variables via my()" in perlsub for details.

	       A local modifies the listed variables to be local to the
	       enclosing block, file, or eval.	If more than one value is
	       listed, the list must be placed in parentheses.	See "Temporary
	       Values via local()" in perlsub for details, including issues
	       with tied arrays and hashes.

       localtime EXPR
       localtime
	       Converts a time as returned by the time function to a 9-element
	       list with the time analyzed for the local time zone.  Typically
	       used as follows:

		   #  0    1	2     3     4	 5     6     7	   8
		   ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
							       localtime(time);

	       All list elements are numeric, and come straight out of the C
	       `struct tm'.  $sec, $min, and $hour are the seconds, minutes,
	       and hours of the specified time.

	       $mday is the day of the month, and $mon is the month itself, in
	       the range 0..11 with 0 indicating January and 11 indicating
	       December.  This makes it easy to get a month name from a list:

		   my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
		   print "$abbr[$mon] $mday";
		   # $mon=9, $mday=18 gives "Oct 18"

	       $year is the number of years since 1900, not just the last two
	       digits of the year.  That is, $year is 123 in year 2023.  The
	       proper way to get a complete 4-digit year is simply:

		   $year += 1900;

	       To get the last two digits of the year (e.g., '01' in 2001) do:

		   $year = sprintf("%02d", $year % 100);

	       $wday is the day of the week, with 0 indicating Sunday and 3
	       indicating Wednesday.  $yday is the day of the year, in the
	       range 0..364 (or 0..365 in leap years.)

	       $isdst is true if the specified time occurs during Daylight
	       Saving Time, false otherwise.

	       If EXPR is omitted, "localtime()" uses the current time
	       ("localtime(time)").

	       In scalar context, "localtime()" returns the ctime(3) value:

		   $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

	       This scalar value is not locale dependent but is a Perl
	       builtin. For GMT instead of local time use the "gmtime"
	       builtin. See also the "Time::Local" module (to convert the sec-
	       ond, minutes, hours, ... back to the integer value returned by
	       time()), and the POSIX module's strftime(3) and mktime(3) func-
	       tions.

	       To get somewhat similar but locale dependent date strings, set
	       up your locale environment variables appropriately (please see
	       perllocale) and try for example:

		   use POSIX qw(strftime);
		   $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
		   # or for GMT formatted appropriately for your locale:
		   $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

	       Note that the %a and %b, the short forms of the day of the week
	       and the month of the year, may not necessarily be three charac-
	       ters wide.

	       See "localtime" in perlport for portability concerns.

       lock THING
	       This function places an advisory lock on a shared variable, or
	       referenced object contained in THING until the lock goes out of
	       scope.

	       lock() is a "weak keyword" : this means that if you've defined
	       a function by this name (before any calls to it), that function
	       will be called instead. (However, if you've said "use threads",
	       lock() is always a keyword.) See threads.

       log EXPR
       log     Returns the natural logarithm (base e) of EXPR.	If EXPR is
	       omitted, returns log of $_.  To get the log of another base,
	       use basic algebra: The base-N log of a number is equal to the
	       natural log of that number divided by the natural log of N.
	       For example:

		   sub log10 {
		       my $n = shift;
		       return log($n)/log(10);
		   }

	       See also "exp" for the inverse operation.

       lstat EXPR
       lstat   Does the same thing as the "stat" function (including setting
	       the special "_" filehandle) but stats a symbolic link instead
	       of the file the symbolic link points to.  If symbolic links are
	       unimplemented on your system, a normal "stat" is done.  For
	       much more detailed information, please see the documentation
	       for "stat".

	       If EXPR is omitted, stats $_.

       m//     The match operator.  See perlop.

       map BLOCK LIST
       map EXPR,LIST
	       Evaluates the BLOCK or EXPR for each element of LIST (locally
	       setting $_ to each element) and returns the list value composed
	       of the results of each such evaluation.	In scalar context,
	       returns the total number of elements so generated.  Evaluates
	       BLOCK or EXPR in list context, so each element of LIST may pro-
	       duce zero, one, or more elements in the returned value.

		   @chars = map(chr, @nums);

	       translates a list of numbers to the corresponding characters.
	       And

		   %hash = map { getkey($_) => $_ } @array;

	       is just a funny way to write

		   %hash = ();
		   foreach $_ (@array) {
		       $hash{getkey($_)} = $_;
		   }

	       Note that $_ is an alias to the list value, so it can be used
	       to modify the elements of the LIST.  While this is useful and
	       supported, it can cause bizarre results if the elements of LIST
	       are not variables.  Using a regular "foreach" loop for this
	       purpose would be clearer in most cases.	See also "grep" for an
	       array composed of those items of the original list for which
	       the BLOCK or EXPR evaluates to true.

	       "{" starts both hash references and blocks, so "map { ..."
	       could be either the start of map BLOCK LIST or map EXPR, LIST.
	       Because perl doesn't look ahead for the closing "}" it has to
	       take a guess at which its dealing with based what it finds just
	       after the "{". Usually it gets it right, but if it doesn't it
	       won't realize something is wrong until it gets to the "}" and
	       encounters the missing (or unexpected) comma. The syntax error
	       will be reported close to the "}" but you'll need to change
	       something near the "{" such as using a unary "+" to give perl
	       some help:

		   %hash = map {  "\L$_", 1  } @array  # perl guesses EXPR.  wrong
		   %hash = map { +"\L$_", 1  } @array  # perl guesses BLOCK. right
		   %hash = map { ("\L$_", 1) } @array  # this also works
		   %hash = map {  lc($_), 1  } @array  # as does this.
		   %hash = map +( lc($_), 1 ), @array  # this is EXPR and works!

		   %hash = map	( lc($_), 1 ), @array  # evaluates to (1, @array)

	       or to force an anon hash constructor use "+{"

		  @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end

	       and you get list of anonymous hashes each with only 1 entry.

       mkdir FILENAME,MASK
       mkdir FILENAME
	       Creates the directory specified by FILENAME, with permissions
	       specified by MASK (as modified by "umask").  If it succeeds it
	       returns true, otherwise it returns false and sets $! (errno).
	       If omitted, MASK defaults to 0777.

	       In general, it is better to create directories with permissive
	       MASK, and let the user modify that with their "umask", than it
	       is to supply a restrictive MASK and give the user no way to be
	       more permissive.  The exceptions to this rule are when the file
	       or directory should be kept private (mail files, for instance).
	       The perlfunc(1) entry on "umask" discusses the choice of MASK
	       in more detail.

	       Note that according to the POSIX 1003.1-1996 the FILENAME may
	       have any number of trailing slashes.  Some operating and
	       filesystems do not get this right, so Perl automatically
	       removes all trailing slashes to keep everyone happy.

       msgctl ID,CMD,ARG
	       Calls the System V IPC function msgctl(2).  You'll probably
	       have to say

		   use IPC::SysV;

	       first to get the correct constant definitions.  If CMD is
	       "IPC_STAT", then ARG must be a variable that will hold the
	       returned "msqid_ds" structure.  Returns like "ioctl": the unde-
	       fined value for error, "0 but true" for zero, or the actual
	       return value otherwise.	See also "SysV IPC" in perlipc,
	       "IPC::SysV", and "IPC::Semaphore" documentation.

       msgget KEY,FLAGS
	       Calls the System V IPC function msgget(2).  Returns the message
	       queue id, or the undefined value if there is an error.  See
	       also "SysV IPC" in perlipc and "IPC::SysV" and "IPC::Msg" docu-
	       mentation.

       msgrcv ID,VAR,SIZE,TYPE,FLAGS
	       Calls the System V IPC function msgrcv to receive a message
	       from message queue ID into variable VAR with a maximum message
	       size of SIZE.  Note that when a message is received, the mes-
	       sage type as a native long integer will be the first thing in
	       VAR, followed by the actual message.  This packing may be
	       opened with "unpack("l! a*")".  Taints the variable.  Returns
	       true if successful, or false if there is an error.  See also
	       "SysV IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Msg" docu-
	       mentation.

       msgsnd ID,MSG,FLAGS
	       Calls the System V IPC function msgsnd to send the message MSG
	       to the message queue ID.  MSG must begin with the native long
	       integer message type, and be followed by the length of the
	       actual message, and finally the message itself.	This kind of
	       packing can be achieved with "pack("l! a*", $type, $message)".
	       Returns true if successful, or false if there is an error.  See
	       also "IPC::SysV" and "IPC::SysV::Msg" documentation.

       my EXPR
       my TYPE EXPR
       my EXPR : ATTRS
       my TYPE EXPR : ATTRS
	       A "my" declares the listed variables to be local (lexically) to
	       the enclosing block, file, or "eval".  If more than one value
	       is listed, the list must be placed in parentheses.

	       The exact semantics and interface of TYPE and ATTRS are still
	       evolving.  TYPE is currently bound to the use of "fields"
	       pragma, and attributes are handled using the "attributes"
	       pragma, or starting from Perl 5.8.0 also via the
	       "Attribute::Handlers" module.  See "Private Variables via my()"
	       in perlsub for details, and fields, attributes, and
	       Attribute::Handlers.

       next LABEL
       next    The "next" command is like the "continue" statement in C; it
	       starts the next iteration of the loop:

		   LINE: while () {
		       next LINE if /^#/;      # discard comments
		       #...
		   }

	       Note that if there were a "continue" block on the above, it
	       would get executed even on discarded lines.  If the LABEL is
	       omitted, the command refers to the innermost enclosing loop.

	       "next" cannot be used to exit a block which returns a value
	       such as "eval {}", "sub {}" or "do {}", and should not be used
	       to exit a grep() or map() operation.

	       Note that a block by itself is semantically identical to a loop
	       that executes once.  Thus "next" will exit such a block early.

	       See also "continue" for an illustration of how "last", "next",
	       and "redo" work.

       no Module VERSION LIST
       no Module VERSION
       no Module LIST
       no Module
	       See the "use" function, which "no" is the opposite of.

       oct EXPR
       oct     Interprets EXPR as an octal string and returns the correspond-
	       ing value.  (If EXPR happens to start off with "0x", interprets
	       it as a hex string.  If EXPR starts off with "0b", it is inter-
	       preted as a binary string.  Leading whitespace is ignored in
	       all three cases.)  The following will handle decimal, binary,
	       octal, and hex in the standard Perl or C notation:

		   $val = oct($val) if $val =~ /^0/;

	       If EXPR is omitted, uses $_.   To go the other way (produce a
	       number in octal), use sprintf() or printf():

		   $perms = (stat("filename"))[2] & 07777;
		   $oct_perms = sprintf "%lo", $perms;

	       The oct() function is commonly used when a string such as 644
	       needs to be converted into a file mode, for example. (Although
	       perl will automatically convert strings into numbers as needed,
	       this automatic conversion assumes base 10.)

       open FILEHANDLE,EXPR
       open FILEHANDLE,MODE,EXPR
       open FILEHANDLE,MODE,EXPR,LIST
       open FILEHANDLE,MODE,REFERENCE
       open FILEHANDLE
	       Opens the file whose filename is given by EXPR, and associates
	       it with FILEHANDLE.

	       (The following is a comprehensive reference to open(): for a
	       gentler introduction you may consider perlopentut.)

	       If FILEHANDLE is an undefined scalar variable (or array or hash
	       element) the variable is assigned a reference to a new anony-
	       mous filehandle, otherwise if FILEHANDLE is an expression, its
	       value is used as the name of the real filehandle wanted.  (This
	       is considered a symbolic reference, so "use strict 'refs'"
	       should not be in effect.)

	       If EXPR is omitted, the scalar variable of the same name as the
	       FILEHANDLE contains the filename.  (Note that lexical vari-
	       ables--those declared with "my"--will not work for this pur-
	       pose; so if you're using "my", specify EXPR in your call to
	       open.)

	       If three or more arguments are specified then the mode of open-
	       ing and the file name are separate. If MODE is '<' or nothing,
	       the file is opened for input.  If MODE is '>', the file is
	       truncated and opened for output, being created if necessary.
	       If MODE is '>>', the file is opened for appending, again being
	       created if necessary.

	       You can put a '+' in front of the '>' or '<' to indicate that
	       you want both read and write access to the file; thus '+<' is
	       almost always preferred for read/write updates--the '+>' mode
	       would clobber the file first.  You can't usually use either
	       read-write mode for updating textfiles, since they have vari-
	       able length records.  See the -i switch in perlrun for a better
	       approach.  The file is created with permissions of 0666 modi-
	       fied by the process' "umask" value.

	       These various prefixes correspond to the fopen(3) modes of 'r',
	       'r+', 'w', 'w+', 'a', and 'a+'.

	       In the 2-arguments (and 1-argument) form of the call the mode
	       and filename should be concatenated (in this order), possibly
	       separated by spaces.  It is possible to omit the mode in these
	       forms if the mode is '<'.

	       If the filename begins with '|', the filename is interpreted as
	       a command to which output is to be piped, and if the filename
	       ends with a '|', the filename is interpreted as a command which
	       pipes output to us.  See "Using open() for IPC" in perlipc for
	       more examples of this.  (You are not allowed to "open" to a
	       command that pipes both in and out, but see IPC::Open2,
	       IPC::Open3, and "Bidirectional Communication with Another
	       Process" in perlipc for alternatives.)

	       For three or more arguments if MODE is '|-', the filename is
	       interpreted as a command to which output is to be piped, and if
	       MODE is '-|', the filename is interpreted as a command which
	       pipes output to us.  In the 2-arguments (and 1-argument) form
	       one should replace dash ('-') with the command.	See "Using
	       open() for IPC" in perlipc for more examples of this.  (You are
	       not allowed to "open" to a command that pipes both in and out,
	       but see IPC::Open2, IPC::Open3, and "Bidirectional Communica-
	       tion" in perlipc for alternatives.)

	       In the three-or-more argument form of pipe opens, if LIST is
	       specified (extra arguments after the command name) then LIST
	       becomes arguments to the command invoked if the platform sup-
	       ports it.  The meaning of "open" with more than three arguments
	       for non-pipe modes is not yet specified. Experimental "layers"
	       may give extra LIST arguments meaning.

	       In the 2-arguments (and 1-argument) form opening '-' opens
	       STDIN and opening '>-' opens STDOUT.

	       You may use the three-argument form of open to specify IO "lay-
	       ers" (sometimes also referred to as "disciplines") to be
	       applied to the handle that affect how the input and output are
	       processed (see open and PerlIO for more details). For example

		 open(FH, "<:utf8", "file")

	       will open the UTF-8 encoded file containing Unicode characters,
	       see perluniintro. Note that if layers are specified in the
	       three-arg form then default layers stored in ${^OPEN} (see per-
	       lvar; usually set by the open pragma or the switch -CioD) are
	       ignored.

	       Open returns nonzero upon success, the undefined value other-
	       wise.  If the "open" involved a pipe, the return value happens
	       to be the pid of the subprocess.

	       If you're running Perl on a system that distinguishes between
	       text files and binary files, then you should check out "bin-
	       mode" for tips for dealing with this.  The key distinction
	       between systems that need "binmode" and those that don't is
	       their text file formats.  Systems like Unix, Mac OS, and Plan
	       9, which delimit lines with a single character, and which
	       encode that character in C as "\n", do not need "binmode".  The
	       rest need it.

	       When opening a file, it's usually a bad idea to continue normal
	       execution if the request failed, so "open" is frequently used
	       in connection with "die".  Even if "die" won't do what you want
	       (say, in a CGI script, where you want to make a nicely format-
	       ted error message (but there are modules that can help with
	       that problem)) you should always check the return value from
	       opening a file.	The infrequent exception is when working with
	       an unopened filehandle is actually what you want to do.

	       As a special case the 3-arg form with a read/write mode and the
	       third argument being "undef":

		   open(TMP, "+>", undef) or die ...

	       opens a filehandle to an anonymous temporary file.  Also using
	       "+<" works for symmetry, but you really should consider writing
	       something to the temporary file first.  You will need to seek()
	       to do the reading.

	       Since v5.8.0, perl has built using PerlIO by default.  Unless
	       you've changed this (i.e. Configure -Uuseperlio), you can open
	       file handles to "in memory" files held in Perl scalars via:

		   open($fh, '>', \$variable) || ..

	       Though if you try to re-open "STDOUT" or "STDERR" as an "in
	       memory" file, you have to close it first:

		   close STDOUT;
		   open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";

	       Examples:

		   $ARTICLE = 100;
		   open ARTICLE or die "Can't find article $ARTICLE: $!\n";
		   while (
) {... open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) # if the open fails, output is discarded open(DBASE, '+<', 'dbase.mine') # open for update or die "Can't open 'dbase.mine' for update: $!"; open(DBASE, '+Tmp$$") # $$ is our process id or die "Can't start sort: $!"; # in memory files open(MEMORY,'>', \$var) or die "Can't open memory file: $!"; print MEMORY "foo!\n"; # output will end up in $var # process argument list of files along with any includes foreach $file (@ARGV) { process($file, 'fh00'); } sub process { my($filename, $input) = @_; $input++; # this is a string increment unless (open($input, $filename)) { print STDERR "Can't open $filename: $!\n"; return; } local $_; while (<$input>) { # note use of indirection if (/^#include "(.*)"/) { process($1, $input); next; } #... # whatever } } See perliol for detailed info on PerlIO. You may also, in the Bourne shell tradition, specify an EXPR beginning with '>&', in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped (as dup(2)) and opened. You may use "&" after ">", ">>", "<", "+>", "+>>", and "+<". The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers.) If you use the 3-arg form then you can pass either a number, the name of a filehandle or the normal "reference to a glob". Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using various methods: #!/usr/bin/perl open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!"; open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!"; open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!"; open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!"; select STDERR; $| = 1; # make unbuffered select STDOUT; $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!"; open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!"; print STDOUT "stdout 2\n"; print STDERR "stderr 2\n"; If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then Perl will do an equivalent of C's "fdopen" of that file descriptor (and not call dup(2)); this is more parsimonious of file descriptors. For example: # open for input, reusing the fileno of $fd open(FILEHANDLE, "<&=$fd") or open(FILEHANDLE, "<&=", $fd) or # open for append, using the fileno of OLDFH open(FH, ">>&=", OLDFH) or open(FH, ">>&=OLDFH") Being parsimonious on filehandles is also useful (besides being parsimonious) for example when something is dependent on file descriptors, like for example locking using flock(). If you do just "open(A, '>>&B')", the filehandle A will not have the same file descriptor as B, and therefore flock(A) will not flock(B), and vice versa. But with "open(A, '>>&=B')" the filehandles will share the same file descriptor. Note that if you are using Perls older than 5.8.0, Perl will be using the standard C libraries' fdopen() to implement the "=" functionality. On many UNIX systems fdopen() fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is most often the default. You can see whether Perl has been compiled with PerlIO or not by running "perl -V" and looking for "useperlio=" line. If "useperlio" is "define", you have PerlIO, otherwise you don't. If you open a pipe on the command '-', i.e., either '|-' or '-|' with 2-arguments (or 1-argument) form of open(), then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use "defined($pid)" to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe com- mand gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following triples are more or less equivalent: open(FOO, "|tr '[a-z]' '[A-Z]'"); open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; open(FOO, '|-', "tr", '[a-z]', '[A-Z]'); open(FOO, "cat -n '$file'|"); open(FOO, '-|', "cat -n '$file'"); open(FOO, '-|') || exec 'cat', '-n', $file; open(FOO, '-|', "cat", '-n', $file); The last example in each block shows the pipe as "list form", which is not yet supported on all platforms. A good rule of thumb is that if your platform has true "fork()" (in other words, if your platform is UNIX) you can use the list form. See "Safe Pipe Opens" in perlipc for more examples of this. Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See "$^F" in perlvar. Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $?. The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of "rsh cat file |", or you could change certain filenames as needed: $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; open(FH, $filename) or die "Can't open $filename: $!"; Use 3-argument form to open a file with arbitrary weird charac- ters in it, open(FOO, '<', $file); otherwise it's necessary to protect any leading and trailing whitespace: $file =~ s#^(\s)#./$1#; open(FOO, "< $file\0"); (this may not work on some bizarre filesystems). One should conscientiously choose between the magic and 3-arguments form of open(): open IN, $ARGV[0]; will allow the user to specify an argument of the form "rsh cat file |", but will not work on a filename which happens to have a trailing space, while open IN, '<', $ARGV[0]; will have exactly the opposite restrictions. If you want a "real" C "open" (see open(2) on your system), then you should use the "sysopen" function, which involves no such magic (but may use subtly different filemodes than Perl open(), which is mapped to C fopen()). This is another way to protect your filenames from interpretation. For example: use IO::Handle; sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) or die "sysopen $path: $!"; $oldfh = select(HANDLE); $| = 1; select($oldfh); print HANDLE "stuff $$\n"; seek(HANDLE, 0, 0); print "File contains: ", ; Using the constructor from the "IO::Handle" package (or one of its subclasses, such as "IO::File" or "IO::Socket"), you can generate anonymous filehandles that have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope: use IO::File; #... sub read_myfile_munged { my $ALL = shift; my $handle = new IO::File; open($handle, "myfile") or die "myfile: $!"; $first = <$handle> or return (); # Automatically closed here. mung $first or die "mung failed"; # Or here. return $first, <$handle> if $ALL; # Or here. $first; # Or here. } See "seek" for some details about mixing reading and writing. opendir DIRHANDLE,EXPR Opens a directory named EXPR for processing by "readdir", "telldir", "seekdir", "rewinddir", and "closedir". Returns true if successful. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhan- dle name. If DIRHANDLE is an undefined scalar variable (or array or hash element), the variable is assigned a reference to a new anonymous dirhandle. DIRHANDLEs have their own namespace separate from FILEHANDLEs. ord EXPR ord Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, or Unicode) value of the first character of EXPR. If EXPR is omitted, uses $_. For the reverse, see "chr". See perlunicode and encoding for more about Unicode. our EXPR our EXPR TYPE our EXPR : ATTRS our TYPE EXPR : ATTRS "our" associates a simple name with a package variable in the current package for use within the current scope. When "use strict 'vars'" is in effect, "our" lets you use declared global variables without qualifying them with package names, within the lexical scope of the "our" declaration. In this way "our" differs from "use vars", which is package scoped. Unlike "my", which both allocates storage for a variable and associates a simple name with that storage for use within the current scope, "our" associates a simple name with a package variable in the current package, for use within the current scope. In other words, "our" has the same scoping rules as "my", but does not necessarily create a variable. If more than one value is listed, the list must be placed in parentheses. our $foo; our($bar, $baz); An "our" declaration declares a global variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is entered is determined at the point of the declaration, not at the point of use. This means the following behavior holds: package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; print $bar; # prints 20, as it refers to $Foo::bar Multiple "our" declarations with the same name in the same lex- ical scope are allowed if they are in different packages. If they happen to be in the same package, Perl will emit warnings if you have asked for them, just like multiple "my" declara- tions. Unlike a second "my" declaration, which will bind the name to a fresh variable, a second "our" declaration in the same package, in the same scope, is merely redundant. use warnings; package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; our $bar = 30; # declares $Bar::bar for rest of lexical scope print $bar; # prints 30 our $bar; # emits warning but has no other effect print $bar; # still prints 30 An "our" declaration may also have a list of attributes associ- ated with it. The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See "Private Variables via my()" in perlsub for details, and fields, attributes, and Attribute::Handlers. The only currently recognized "our()" attribute is "unique" which indicates that a single copy of the global is to be used by all interpreters should the program happen to be running in a multi-interpreter environment. (The default behaviour would be for each interpreter to have its own copy of the global.) Examples: our @EXPORT : unique = qw(foo); our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]); our $VERSION : unique = "1.00"; Note that this attribute also has the effect of making the global readonly when the first new interpreter is cloned (for example, when the first new thread is created). Multi-interpreter environments can come to being either through the fork() emulation on Windows platforms, or by embedding perl in a multi-threaded application. The "unique" attribute does nothing in all other environments. Warning: the current implementation of this attribute operates on the typeglob associated with the variable; this means that "our $x : unique" also has the effect of "our @x : unique; our %x : unique". This may be subject to change. pack TEMPLATE,LIST Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE. The resulting string is the con- catenation of the converted values. Typically, each converted value looks like its machine-level representation. For exam- ple, on 32-bit machines a converted integer may be represented by a sequence of 4 bytes. The TEMPLATE is a sequence of characters that give the order and type of values, as follows: a A string with arbitrary binary data, will be null padded. A A text (ASCII) string, will be space padded. Z A null terminated (ASCIZ) string, will be null padded. b A bit string (ascending bit order inside each byte, like vec()). B A bit string (descending bit order inside each byte). h A hex string (low nybble first). H A hex string (high nybble first). c A signed char value. C An unsigned char value. Only does bytes. See U for Unicode. s A signed short value. S An unsigned short value. (This 'short' is _exactly_ 16 bits, which may differ from what a local C compiler calls 'short'. If you want native-length shorts, use the '!' suffix.) i A signed integer value. I An unsigned integer value. (This 'integer' is _at_least_ 32 bits wide. Its exact size depends on what a local C compiler calls 'int', and may even be larger than the 'long' described in the next item.) l A signed long value. L An unsigned long value. (This 'long' is _exactly_ 32 bits, which may differ from what a local C compiler calls 'long'. If you want native-length longs, use the '!' suffix.) n An unsigned short in "network" (big-endian) order. N An unsigned long in "network" (big-endian) order. v An unsigned short in "VAX" (little-endian) order. V An unsigned long in "VAX" (little-endian) order. (These 'shorts' and 'longs' are _exactly_ 16 bits and _exactly_ 32 bits, respectively.) q A signed quad (64-bit) value. Q An unsigned quad value. (Quads are available only if your system supports 64-bit integer values _and_ if Perl has been compiled to support those. Causes a fatal error otherwise.) j A signed integer value (a Perl internal integer, IV). J An unsigned integer value (a Perl internal unsigned integer, UV). f A single-precision float in the native format. d A double-precision float in the native format. F A floating point value in the native native format (a Perl internal floating point value, NV). D A long double-precision float in the native format. (Long doubles are available only if your system supports long double values _and_ if Perl has been compiled to support those. Causes a fatal error otherwise.) p A pointer to a null-terminated string. P A pointer to a structure (fixed-length string). u A uuencoded string. U A Unicode character number. Encodes to UTF-8 internally (or UTF-EBCDIC in EBCDIC platforms). w A BER compressed integer (not an ASN.1 BER, see perlpacktut for details). Its bytes represent an unsigned integer in base 128, most significant digit first, with as few digits as possible. Bit eight (the high bit) is set on each byte except the last. x A null byte. X Back up a byte. @ Null fill to absolute position, counted from the start of the innermost ()-group. ( Start of a ()-group. The following rules apply: * Each letter may optionally be followed by a number giv- ing a repeat count. With all types except "a", "A", "Z", "b", "B", "h", "H", "@", "x", "X" and "P" the pack function will gobble up that many values from the LIST. A "*" for the repeat count means to use however many items are left, except for "@", "x", "X", where it is equivalent to 0, and "u", where it is equivalent to 1 (or 45, what is the same). A numeric repeat count may optionally be enclosed in brackets, as in "pack 'C[80]', @arr". One can replace the numeric repeat count by a template enclosed in brackets; then the packed length of this template in bytes is used as a count. For example, "x[L]" skips a long (it skips the number of bytes in a long); the template "$t X[$t] $t" unpack()s twice what $t unpacks. If the template in brackets contains alignment commands (such as "x![d]"), its packed length is calculated as if the start of the template has the maximal possible alignment. When used with "Z", "*" results in the addition of a trailing null byte (so the packed result will be one longer than the byte "length" of the item). The repeat count for "u" is interpreted as the maximal number of bytes to encode per line of output, with 0 and 1 replaced by 45. * The "a", "A", and "Z" types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as necessary. When unpacking, "A" strips trailing spaces and nulls, "Z" strips everything after the first null, and "a" returns data verbatim. When packing, "a", and "Z" are equivalent. If the value-to-pack is too long, it is truncated. If too long and an explicit count is provided, "Z" packs only "$count-1" bytes, followed by a null byte. Thus "Z" always packs a trailing null byte under all circum- stances. * Likewise, the "b" and "B" fields pack a string that many bits long. Each byte of the input field of pack() generates 1 bit of the result. Each result bit is based on the least-significant bit of the corresponding input byte, i.e., on "ord($byte)%2". In particular, bytes "0" and "1" generate bits 0 and 1, as do bytes "\0" and "\1". Starting from the beginning of the input string of pack(), each 8-tuple of bytes is converted to 1 byte of output. With format "b" the first byte of the 8-tuple determines the least-significant bit of a byte, and with format "B" it determines the most-significant bit of a byte. If the length of the input string is not exactly divis- ible by 8, the remainder is packed as if the input string were padded by null bytes at the end. Simi- larly, during unpack()ing the "extra" bits are ignored. If the input string of pack() is longer than needed, extra bytes are ignored. A "*" for the repeat count of pack() means to use all the bytes of the input field. On unpack()ing the bits are converted to a string of "0"s and "1"s. * The "h" and "H" fields pack a string that many nybbles (4-bit groups, representable as hexadecimal digits, 0-9a-f) long. Each byte of the input field of pack() generates 4 bits of the result. For non-alphabetical bytes the result is based on the 4 least-significant bits of the input byte, i.e., on "ord($byte)%16". In particular, bytes "0" and "1" generate nybbles 0 and 1, as do bytes "\0" and "\1". For bytes "a".."f" and "A".."F" the result is compatible with the usual hexadecimal digits, so that "a" and "A" both generate the nybble "0xa==10". The result for bytes "g".."z" and "G".."Z" is not well-defined. Starting from the beginning of the input string of pack(), each pair of bytes is converted to 1 byte of output. With format "h" the first byte of the pair determines the least-significant nybble of the output byte, and with format "H" it determines the most-sig- nificant nybble. If the length of the input string is not even, it behaves as if padded by a null byte at the end. Simi- larly, during unpack()ing the "extra" nybbles are ignored. If the input string of pack() is longer than needed, extra bytes are ignored. A "*" for the repeat count of pack() means to use all the bytes of the input field. On unpack()ing the bits are converted to a string of hexadecimal digits. * The "p" type packs a pointer to a null-terminated string. You are responsible for ensuring the string is not a temporary value (which can potentially get deal- located before you get around to using the packed result). The "P" type packs a pointer to a structure of the size indicated by the length. A NULL pointer is created if the corresponding value for "p" or "P" is "undef", similarly for unpack(). * The "/" template character allows packing and unpacking of strings where the packed structure contains a byte count followed by the string itself. You write length- item"/"string-item. The length-item can be any "pack" template letter, and describes how the length value is packed. The ones likely to be of most use are integer-packing ones like "n" (for Java strings), "w" (for ASN.1 or SNMP) and "N" (for Sun XDR). For "pack", the string-item must, at present, be "A*", "a*" or "Z*". For "unpack" the length of the string is obtained from the length-item, but if you put in the '*' it will be ignored. For all other codes, "unpack" applies the length value to the next item, which must not have a repeat count. unpack 'C/a', "\04Gurusamy"; gives 'Guru' unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" The length-item is not returned explicitly from "unpack". Adding a count to the length-item letter is unlikely to do anything useful, unless that letter is "A", "a" or "Z". Packing with a length-item of "a" or "Z" may introduce "\000" characters, which Perl does not regard as legal in numeric strings. * The integer types "s", "S", "l", and "L" may be immedi- ately followed by a "!" suffix to signify native shorts or longs--as you can see from above for example a bare "l" does mean exactly 32 bits, the native "long" (as seen by the local C compiler) may be larger. This is an issue mainly in 64-bit platforms. You can see whether using "!" makes any difference by print length(pack("s")), " ", length(pack("s!")), "\n"; print length(pack("l")), " ", length(pack("l!")), "\n"; "i!" and "I!" also work but only because of complete- ness; they are identical to "i" and "I". The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available via Config: use Config; print $Config{shortsize}, "\n"; print $Config{intsize}, "\n"; print $Config{longsize}, "\n"; print $Config{longlongsize}, "\n"; (The $Config{longlongsize} will be undefined if your system does not support long longs.) * The integer formats "s", "S", "i", "I", "l", "L", "j", and "J" are inherently non-portable between processors and operating systems because they obey the native byteorder and endianness. For example a 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively (arranged in and handled by the CPU registers) into bytes as 0x12 0x34 0x56 0x78 # big-endian 0x78 0x56 0x34 0x12 # little-endian Basically, the Intel and VAX CPUs are little-endian, while everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses them in big- endian mode. The names `big-endian' and `little-endian' are comic references to the classic "Gulliver's Travels" (via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and the egg-eat- ing habits of the Lilliputians. Some systems may have even weirder byte orders such as 0x56 0x78 0x12 0x34 0x34 0x12 0x78 0x56 You can see your system's preference with print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack("L",0x12345678))), "\n"; The byteorder on the platform where Perl was built is also available via Config: use Config; print $Config{byteorder}, "\n"; Byteorders '1234' and '12345678' are little-endian, '4321' and '87654321' are big-endian. If you want portable packed integers use the formats "n", "N", "v", and "V", their byte endianness and size are known. See also perlport. * Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of float- ing formats around, and the lack of a standard "net- work" representation, no facility for interchange has been made. This means that packed floating point data written on one machine may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). See also perlport. Note that Perl uses doubles internally for all numeric calculation, and converting from double into float and thence back to double again will lose precision (i.e., "unpack("f", pack("f", $foo)") will not in general equal $foo). * If the pattern begins with a "U", the resulting string will be treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string with an initial "U0", and the bytes that follow will be interpreted as Unicode characters. If you don't want this to happen, you can begin your pattern with "C0" (or anything else) to force Perl not to UTF-8 encode your string, and then follow this with a "U*" somewhere in your pattern. * You must yourself do any alignment or padding by inserting for example enough 'x'es while packing. There is no way to pack() and unpack() could know where the bytes are going to or coming from. Therefore "pack" (and "unpack") handle their output and input as flat sequences of bytes. * A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count, both as postfix, and for unpack() also via the "/" template character. Within each repetition of a group, positioning with "@" starts again at 0. Therefore, the result of pack( '@1A((@2A)@3A)', 'a', 'b', 'c' ) is the string "\0a\0\0bc". * "x" and "X" accept "!" modifier. In this case they act as alignment commands: they jump forward/back to the closest position aligned at a multiple of "count" bytes. For example, to pack() or unpack() C's "struct {char c; double d; char cc[2]}" one may need to use the template "C x![d] d C[2]"; this assumes that doubles must be aligned on the double's size. For alignment commands "count" of 0 is equivalent to "count" of 1; both result in no-ops. * A comment in a TEMPLATE starts with "#" and goes to the end of line. White space may be used to separate pack codes from each other, but a "!" modifier and a repeat count must follow immediately. * If TEMPLATE requires more arguments to pack() than actually given, pack() assumes additional "" arguments. If TEMPLATE requires fewer arguments to pack() than actually given, extra arguments are ignored. Examples: $foo = pack("CCCC",65,66,67,68); # foo eq "ABCD" $foo = pack("C4",65,66,67,68); # same thing $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); # same thing with Unicode circled letters $foo = pack("ccxxcc",65,66,67,68); # foo eq "AB\0\0CD" # note: the above examples featuring "C" and "c" are true # only on ASCII and ASCII-derived systems such as ISO Latin 1 # and UTF-8. In EBCDIC the first example would be # $foo = pack("CCCC",193,194,195,196); $foo = pack("s2",1,2); # "\1\0\2\0" on little-endian # "\0\1\0\2" on big-endian $foo = pack("a4","abcd","x","y","z"); # "abcd" $foo = pack("aaaa","abcd","x","y","z"); # "axyz" $foo = pack("a14","abcdefg"); # "abcdefg\0\0\0\0\0\0\0" $foo = pack("i9pl", gmtime); # a real struct tm (on my system anyway) $utmp_template = "Z8 Z8 Z16 L"; $utmp = pack($utmp_template, @utmp1); # a struct utmp (BSDish) @utmp2 = unpack($utmp_template, $utmp); # "@utmp1" eq "@utmp2" sub bintodec { unpack("N", pack("B32", substr("0" x 32 . shift, -32))); } $foo = pack('sx2l', 12, 34); # short 12, two zero bytes padding, long 34 $bar = pack('s@4l', 12, 34); # short 12, zero fill to position 4, long 34 # $foo eq $bar The same template may generally also be used in unpack(). package NAMESPACE package Declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block, file, or eval (the same as the "my" operator). All further unqualified dynamic identifiers will be in this namespace. A package statement affects only dynamic variables--including those you've used "local" on--but not lexical variables, which are created with "my". Typically it would be the first declaration in a file to be included by the "require" or "use" operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the "main" package as assumed. That is, $::sail is equivalent to $main::sail (as well as to $main'sail, still seen in older code). If NAMESPACE is omitted, then there is no current package, and all identifiers must be fully qualified or lexicals. However, you are strongly advised not to make use of this feature. Its use can cause unexpected behaviour, even crashing some versions of Perl. It is deprecated, and will be removed from a future release. See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub for other scoping issues. pipe READHANDLE,WRITEHANDLE Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, dead- lock can occur unless you are very careful. In addition, note that Perl's pipes use IO buffering, so you may need to set $| to flush your WRITEHANDLE after each command, depending on the application. See IPC::Open2, IPC::Open3, and "Bidirectional Communication" in perlipc for examples of such things. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors as determined by the value of $^F. See "$^F" in perlvar. pop ARRAY pop Pops and returns the last value of the array, shortening the array by one element. Has an effect similar to $ARRAY[$#ARRAY--] If there are no elements in the array, returns the undefined value (although this may happen at other times as well). If ARRAY is omitted, pops the @ARGV array in the main program, and the @_ array in subroutines, just like "shift". pos SCALAR pos Returns the offset of where the last "m//g" search left off for the variable in question ($_ is used when the variable is not specified). Note that 0 is a valid match offset. "undef" indicates that the search position is reset (usually due to match failure, but can also be because no match has yet been performed on the scalar). "pos" directly accesses the location used by the regexp engine to store the offset, so assigning to "pos" will change that offset, and so will also influence the "\G" zero-width assertion in regular expressions. Because a failed "m//gc" match doesn't reset the offset, the return from "pos" won't change either in this case. See perlre and perlop. print FILEHANDLE LIST print LIST print Prints a string or a list of strings. Returns true if success- ful. FILEHANDLE may be a scalar variable name, in which case the variable contains the name of or a reference to the file- handle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a "+" or put parentheses around the arguments.) If FILEHANDLE is omit- ted, prints by default to standard output (or to the last selected output channel--see "select"). If LIST is also omit- ted, prints $_ to the currently selected output channel. To set the default output channel to something other than STDOUT use the select operation. The current value of $, (if any) is printed between each LIST item. The current value of "$\" (if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, and any subroutine that you call will have one or more of its expressions evaluated in list context. Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to termi- nate the arguments to the print--interpose a "+" or put paren- theses around all the arguments. Note that if you're storing FILEHANDLEs in an array, or if you're using any other expression more complex than a scalar variable to retrieve it, you will have to use a block returning the filehandle value instead: print { $files[$i] } "stuff\n"; print { $OK ? STDOUT : STDERR } "stuff\n"; printf FILEHANDLE FORMAT, LIST printf FORMAT, LIST Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except that "$\" (the output record separator) is not appended. The first argument of the list will be interpreted as the "printf" format. See "sprintf" for an explanation of the format argu- ment. If "use locale" is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale. Don't fall into the trap of using a "printf" when a simple "print" would do. The "print" is more efficient and less error prone. prototype FUNCTION Returns the prototype of a function as a string (or "undef" if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is a string starting with "CORE::", the rest is taken as a name for Perl builtin. If the builtin is not over- ridable (such as "qw//") or its arguments cannot be expressed by a prototype (such as "system") returns "undef" because the builtin does not really behave like a Perl function. Other- wise, the string describing the equivalent prototype is returned. push ARRAY,LIST , Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as for $value (LIST) { $ARRAY[++$#ARRAY] = $value; } but is more efficient. Returns the number of elements in the array following the completed "push". q/STRING/ qq/STRING/ qr/STRING/ qx/STRING/ qw/STRING/ Generalized quotes. See "Regexp Quote-Like Operators" in per- lop. quotemeta EXPR quotemeta Returns the value of EXPR with all non-"word" characters back- slashed. (That is, all characters not matching "/[A-Za-z_0-9]/" will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the "\Q" escape in double- quoted strings. If EXPR is omitted, uses $_. rand EXPR rand Returns a random fractional number greater than or equal to 0 and less than the value of EXPR. (EXPR should be positive.) If EXPR is omitted, the value 1 is used. Currently EXPR with the value 0 is also special-cased as 1 - this has not been doc- umented before perl 5.8.0 and is subject to change in future versions of perl. Automatically calls "srand" unless "srand" has already been called. See also "srand". Apply "int()" to the value returned by "rand()" if you want random integers instead of random fractional numbers. For example, int(rand(10)) returns a random integer between 0 and 9, inclusive. (Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was prob- ably compiled with the wrong number of RANDBITS.) read FILEHANDLE,SCALAR,LENGTH,OFFSET read FILEHANDLE,SCALAR,LENGTH Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE. Returns the number of charac- ters actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that the last character actually read is the last character of the scalar after the read. An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended. The call is actually implemented in terms of either Perl's or system's fread() call. To get a true read(2) system call, see "sysread". Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default all filehandles operate on bytes, but for example if the filehandle has been opened with the ":utf8" I/O layer (see "open", and the "open" pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the ":encoding" pragma: in that case pretty much any characters can be read. readdir DIRHANDLE Returns the next directory entry for a directory opened by "opendir". If used in list context, returns all the rest of the entries in the directory. If there are no more entries, returns an undefined value in scalar context or a null list in list context. If you're planning to filetest the return values out of a "readdir", you'd better prepend the directory in question. Otherwise, because we didn't "chdir" there, it would have been testing the wrong file. opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); closedir DIR; readline EXPR Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, each call reads and returns the next line, until end-of-file is reached, whereupon the subsequent call returns undef. In list context, reads until end-of-file is reached and returns a list of lines. Note that the notion of "line" used here is however you may have defined it with $/ or $INPUT_RECORD_SEPARATOR). See "$/" in perlvar. When $/ is set to "undef", when readline() is in scalar context (i.e. file slurp mode), and when an empty file is read, it returns '' the first time, followed by "undef" subsequently. This is the internal function implementing the "" opera- tor, but you can use it directly. The "" operator is discussed in more detail in "I/O Operators" in perlop. $line = ; $line = readline(*STDIN); # same thing If readline encounters an operating system error, $! will be set with the corresponding error message. It can be helpful to check $! when you are reading from filehandles you don't trust, such as a tty or a socket. The following example uses the operator form of "readline", and takes the necessary steps to ensure that "readline" was successful. for (;;) { undef $!; unless (defined( $line = <> )) { die $! if $!; last; # reached EOF } # ... } readlink EXPR readlink Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal error. If there is some system error, returns the undefined value and sets $! (errno). If EXPR is omitted, uses $_. readpipe EXPR EXPR is executed as a system command. The collected standard output of the command is returned. In scalar context, it comes back as a single (potentially multi-line) string. In list con- text, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR). This is the internal function implementing the "qx/EXPR/" operator, but you can use it directly. The "qx/EXPR/" operator is discussed in more detail in "I/O Operators" in perlop. recv SOCKET,SCALAR,LENGTH,FLAGS Receives a message on a socket. Attempts to receive LENGTH characters of data into variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name. Returns the address of the sender if SOCKET's protocol supports this; returns an empty string other- wise. If there's an error, returns the undefined value. This call is actually implemented in terms of recvfrom(2) system call. See "UDP: Message Passing" in perlipc for examples. Note the characters: depending on the status of the socket, either (8-bit) bytes or characters are received. By default all sockets operate on bytes, but for example if the socket has been changed using binmode() to operate with the ":utf8" I/O layer (see the "open" pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the ":encoding" pragma: in that case pretty much any characters can be read. redo LABEL redo The "redo" command restarts the loop block without evaluating the conditional again. The "continue" block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. Programs that want to lie to them- selves about what was just input normally use this command: # a simpleminded Pascal comment stripper # (warning: assumes no { or } in strings) LINE: while () { while (s|({.*}.*){.*}|$1 |) {} s|{.*}| |; if (s|{.*| |) { $front = $_; while () { if (/}/) { # end of comment? s|^|$front\{|; redo LINE; } } } print; } "redo" cannot be used to retry a block which returns a value such as "eval {}", "sub {}" or "do {}", and should not be used to exit a grep() or map() operation. Note that a block by itself is semantically identical to a loop that executes once. Thus "redo" inside such a block will effectively turn it into a looping construct. See also "continue" for an illustration of how "last", "next", and "redo" work. ref EXPR ref Returns a non-empty string if EXPR is a reference, the empty string otherwise. If EXPR is not specified, $_ will be used. The value returned depends on the type of thing the reference is a reference to. Builtin types include: SCALAR ARRAY HASH CODE REF GLOB LVALUE If the referenced object has been blessed into a package, then that package name is returned instead. You can think of "ref" as a "typeof" operator. if (ref($r) eq "HASH") { print "r is a reference to a hash.\n"; } unless (ref($r)) { print "r is not a reference at all.\n"; } See also perlref. rename OLDNAME,NEWNAME Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for success, false otherwise. Behavior of this function varies wildly depending on your sys- tem implementation. For example, it will usually not work across file system boundaries, even though the system mv com- mand sometimes compensates for this. Other restrictions include whether it works on directories, open files, or pre- existing files. Check perlport and either the rename(2) man- page or equivalent system documentation for details. require VERSION require EXPR require Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by $_ if EXPR is not supplied. VERSION may be either a numeric argument such as 5.006, which will be compared to $], or a literal of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION). A fatal error is produced at run time if VERSION is greater than the version of the current Perl interpreter. Compare with "use", which can do a similar check at compile time. Specifying VERSION as a literal of the form v5.6.1 should gen- erally be avoided, because it leads to misleading error mes- sages under earlier versions of Perl that do not support this syntax. The equivalent numeric version should be used instead. require v5.6.1; # run time version check require 5.6.1; # ditto require 5.006_001; # ditto; preferred for backwards compatibility Otherwise, "require" demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of "eval". Has semantics similar to the following subroutine: sub require { my ($filename) = @_; if (exists $INC{$filename}) { return 1 if $INC{$filename}; die "Compilation failed in require"; } my ($realfilename,$result); ITER: { foreach $prefix (@INC) { $realfilename = "$prefix/$filename"; if (-f $realfilename) { $INC{$filename} = $realfilename; $result = do $realfilename; last ITER; } } die "Can't find $filename in \@INC"; } if ($@) { $INC{$filename} = undef; die $@; } elsif (!$result) { delete $INC{$filename}; die "$filename did not return true value"; } else { return $result; } } Note that the file will not be included twice under the same specified name. The file must return true as the last statement to indicate successful execution of any initialization code, so it's cus- tomary to end such a file with "1;" unless you're sure it'll return true otherwise. But it's better just to put the "1;", in case you add more statements. If EXPR is a bareword, the require assumes a ".pm" extension and replaces "::" with "/" in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace. In other words, if you try this: require Foo::Bar; # a splendid bareword The require function will actually look for the "Foo/Bar.pm" file in the directories specified in the @INC array. But if you try this: $class = 'Foo::Bar'; require $class; # $class is not a bareword #or require "Foo::Bar"; # not a bareword because of the "" The require function will look for the "Foo::Bar" file in the @INC array and will complain about not finding "Foo::Bar" there. In this case you can do: eval "require $class"; Now that you understand how "require" looks for files in the case of a bareword argument, there is a little extra function- ality going on behind the scenes. Before "require" looks for a ".pm" extension, it will first look for a filename with a ".pmc" extension. A file with this extension is assumed to be Perl bytecode generated by B::Bytecode. If this file is found, and its modification time is newer than a coinciding ".pm" non- compiled file, it will be loaded in place of that non-compiled file ending in a ".pm" extension. You can also insert hooks into the import facility, by putting directly Perl code into the @INC array. There are three forms of hooks: subroutine references, array references and blessed objects. Subroutine references are the simplest case. When the inclu- sion system walks through @INC and encounters a subroutine, this subroutine gets called with two parameters, the first being a reference to itself, and the second the name of the file to be included (e.g. "Foo/Bar.pm"). The subroutine should return "undef" or a filehandle, from which the file to include will be read. If "undef" is returned, "require" will look at the remaining elements of @INC. If the hook is an array reference, its first element must be a subroutine reference. This subroutine is called as above, but the first parameter is the array reference. This enables to pass indirectly some arguments to the subroutine. In other words, you can write: push @INC, \&my_sub; sub my_sub { my ($coderef, $filename) = @_; # $coderef is \&my_sub ... } or: push @INC, [ \&my_sub, $x, $y, ... ]; sub my_sub { my ($arrayref, $filename) = @_; # Retrieve $x, $y, ... my @parameters = @$arrayref[1..$#$arrayref]; ... } If the hook is an object, it must provide an INC method that will be called as above, the first parameter being the object itself. (Note that you must fully qualify the sub's name, as it is always forced into package "main".) Here is a typical code layout: # In Foo.pm package Foo; sub new { ... } sub Foo::INC { my ($self, $filename) = @_; ... } # In the main program push @INC, new Foo(...); Note that these hooks are also permitted to set the %INC entry corresponding to the files they have loaded. See "%INC" in per- lvar. For a yet-more-powerful import facility, see "use" and perlmod. reset EXPR reset Generally used in a "continue" block at the end of a loop to clear variables and reset "??" searches so that they work again. The expression is interpreted as a list of single char- acters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches ("?pattern?") are reset to match again. Resets only variables or searches in the current package. Always returns 1. Exam- ples: reset 'X'; # reset all X variables reset 'a-z'; # reset lower case variables reset; # just reset ?one-time? searches Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays and your %ENV hash. Resets only package variables--lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See "my". return EXPR return Returns from a subroutine, "eval", or "do FILE" with the value given in EXPR. Evaluation of EXPR may be in list, scalar, or void context, depending on how the return value will be used, and the context may vary from one execution to the next (see "wantarray"). If no EXPR is given, returns an empty list in list context, the undefined value in scalar context, and (of course) nothing at all in a void context. (Note that in the absence of an explicit "return", a subrou- tine, eval, or do FILE will automatically return the value of the last expression evaluated.) reverse LIST In list context, returns a list value consisting of the ele- ments of LIST in the opposite order. In scalar context, con- catenates the elements of LIST and returns a string value with all characters in the opposite order. print reverse <>; # line tac, last line first undef $/; # for efficiency of <> print scalar reverse <>; # character tac, last line tsrif Used without arguments in scalar context, reverse() reverses $_. This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the origi- nal hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file. %by_name = reverse %by_address; # Invert the hash rewinddir DIRHANDLE Sets the current position to the beginning of the directory for the "readdir" routine on DIRHANDLE. rindex STR,SUBSTR,POSITION rindex STR,SUBSTR Works just like index() except that it returns the position of the last occurrence of SUBSTR in STR. If POSITION is speci- fied, returns the last occurrence beginning at or before that position. rmdir FILENAME rmdir Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it returns true, otherwise it returns false and sets $! (errno). If FILENAME is omitted, uses $_. s/// The substitution operator. See perlop. scalar EXPR Forces EXPR to be interpreted in scalar context and returns the value of EXPR. @counts = ( scalar @a, scalar @b, scalar @c ); There is no equivalent operator to force an expression to be interpolated in list context because in practice, this is never needed. If you really wanted to do so, however, you could use the construction "@{[ (some expression) ]}", but usually a sim- ple "(some expression)" suffices. Because "scalar" is unary operator, if you accidentally use for EXPR a parenthesized list, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want. The following single statement: print uc(scalar(&foo,$bar)),$baz; is the moral equivalent of these two: &foo; print(uc($bar),$baz); See perlop for more details on unary operators and the comma operator. seek FILEHANDLE,POSITION,WHENCE Sets FILEHANDLE's position, just like the "fseek" call of "stdio". FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position in bytes to POSITION, 1 to set it to the current position plus POSITION, and 2 to set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the file, cur- rent position, end of the file) from the Fcntl module. Returns 1 upon success, 0 otherwise. Note the in bytes: even if the filehandle has been set to oper- ate on characters (for example by using the ":utf8" open layer), tell() will return byte offsets, not character offsets (because implementing that would render seek() and tell() rather slow). If you want to position file for "sysread" or "syswrite", don't use "seek"--buffering makes its effect on the file's system position unpredictable and non-portable. Use "sysseek" instead. Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 ("SEEK_CUR") is useful for not moving the file position: seek(TEST,0,1); This is also useful for applications emulating "tail -f". Once you hit EOF on your read, and then sleep for a while, you might have to stick in a seek() to reset things. The "seek" doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next "" makes Perl try again to read something. We hope. If that doesn't work (some IO implementations are particularly cantankerous), then you may need something more like this: for (;;) { for ($curpos = tell(FILE); $_ = ; $curpos = tell(FILE)) { # search for some stuff and put it into files } sleep($for_a_while); seek(FILE, $curpos, 0); } seekdir DIRHANDLE,POS Sets the current position for the "readdir" routine on DIRHAN- DLE. POS must be a value returned by "telldir". "seekdir" also has the same caveats about possible directory compaction as the corresponding system library routine. select FILEHANDLE select Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a "write" or a "print" without a file- handle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have to set the top of form format for more than one output channel, you might do the following: select(REPORT1); $^ = 'report1_top'; select(REPORT2); $^ = 'report2_top'; FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus: $oldfh = select(STDERR); $| = 1; select($oldfh); Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as: use IO::Handle; STDERR->autoflush(1); select RBITS,WBITS,EBITS,TIMEOUT This calls the select(2) system call with the bit masks speci- fied, which can be constructed using "fileno" and "vec", along these lines: $rin = $win = $ein = ''; vec($rin,fileno(STDIN),1) = 1; vec($win,fileno(STDOUT),1) = 1; $ein = $rin | $win; If you want to select on many filehandles you might wish to write a subroutine: sub fhbits { my(@fhlist) = split(' ',$_[0]); my($bits); for (@fhlist) { vec($bits,fileno($_),1) = 1; } $bits; } $rin = fhbits('STDIN TTY SOCK'); The usual idiom is: ($nfound,$timeleft) = select($rout=$rin, $wout=$win, $eout=$ein, $timeout); or to block until something becomes ready just do this $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); Most systems do not bother to return anything useful in $timeleft, so calling select() in scalar context just returns $nfound. Any of the bit masks can also be undef. The timeout, if speci- fied, is in seconds, which may be fractional. Note: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $time- out. You can effect a sleep of 250 milliseconds this way: select(undef, undef, undef, 0.25); Note that whether "select" gets restarted after signals (say, SIGALRM) is implementation-dependent. See also perlport for notes on the portability of "select". On error, "select" behaves like the select(2) system call : it returns -1 and sets $!. Note: on some Unixes, the select(2) system call may report a socket file descriptor as "ready for reading", when actually no data is available, thus a subsequent read blocks. It can be avoided using always the O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further details. WARNING: One should not attempt to mix buffered I/O (like "read" or ) with "select", except as permitted by POSIX, and even then only on POSIX systems. You have to use "sysread" instead. semctl ID,SEMNUM,CMD,ARG Calls the System V IPC function "semctl". You'll probably have to say use IPC::SysV; first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable that will hold the returned semid_ds structure or semaphore value array. Returns like "ioctl": the undefined value for error, ""0 but true"" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created with "pack("s!",(0)x$nsem)". See also "SysV IPC" in perlipc, "IPC::SysV", "IPC::Semaphore" documentation. semget KEY,NSEMS,FLAGS Calls the System V IPC function semget. Returns the semaphore id, or the undefined value if there is an error. See also "SysV IPC" in perlipc, "IPC::SysV", "IPC::SysV::Semaphore" doc- umentation. semop KEY,OPSTRING Calls the System V IPC function semop to perform semaphore operations such as signalling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with "pack("s!3", $semnum, $semop, $semflag)". The length of OPSTRING implies the number of semaphore operations. Returns true if successful, or false if there is an error. As an example, the following code waits on semaphore $semnum of semaphore id $semid: $semop = pack("s!3", $semnum, -1, 0); die "Semaphore trouble: $!\n" unless semop($semid, $semop); To signal the semaphore, replace "-1" with 1. See also "SysV IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Semaphore" docu- mentation. send SOCKET,MSG,FLAGS,TO send SOCKET,MSG,FLAGS Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes the same flags as the system call of the same name. On unconnected sockets you must specify a destination to send TO, in which case it does a C "sendto". Returns the number of characters sent, or the undefined value if there is an error. The C system call sendmsg(2) is cur- rently unimplemented. See "UDP: Message Passing" in perlipc for examples. Note the characters: depending on the status of the socket, either (8-bit) bytes or characters are sent. By default all sockets operate on bytes, but for example if the socket has been changed using binmode() to operate with the ":utf8" I/O layer (see "open", or the "open" pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Simi- larly for the ":encoding" pragma: in that case pretty much any characters can be sent. setpgrp PID,PGRP Sets the current process group for the specified PID, 0 for the current process. Will produce a fatal error if used on a machine that doesn't implement POSIX setpgid(2) or BSD setp- grp(2). If the arguments are omitted, it defaults to "0,0". Note that the BSD 4.2 version of "setpgrp" does not accept any arguments, so only "setpgrp(0,0)" is portable. See also "POSIX::setsid()". setpriority WHICH,WHO,PRIORITY Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Will produce a fatal error if used on a machine that doesn't implement setpriority(2). setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL Sets the socket option requested. Returns undefined if there is an error. Use integer constants provided by the "Socket" module for LEVEL and OPNAME. Values for LEVEL can also be obtained from getprotobyname. OPTVAL might either be a packed string or an integer. An integer OPTVAL is shorthand for pack("i", OPTVAL). An example disabling the Nagle's algorithm for a socket: use Socket qw(IPPROTO_TCP TCP_NODELAY); setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); shift ARRAY shift Shifts the first value of the array off and returns it, short- ening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the @_ array within the lexical scope of subroutines and formats, and the @ARGV array at file scopes or within the lexical scopes established by the "eval ''", "BEGIN {}", "INIT {}", "CHECK {}", and "END {}" constructs. See also "unshift", "push", and "pop". "shift" and "unshift" do the same thing to the left end of an array that "pop" and "push" do to the right end. shmctl ID,CMD,ARG Calls the System V IPC function shmctl. You'll probably have to say use IPC::SysV; first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be a variable that will hold the returned "shmid_ds" structure. Returns like ioctl: the unde- fined value for error, "0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in perlipc and "IPC::SysV" documentation. shmget KEY,SIZE,FLAGS Calls the System V IPC function shmget. Returns the shared memory segment id, or the undefined value if there is an error. See also "SysV IPC" in perlipc and "IPC::SysV" documentation. shmread ID,VAR,POS,SIZE shmwrite ID,STRING,POS,SIZE Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable that will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Return true if suc- cessful, or false if there is an error. shmread() taints the variable. See also "SysV IPC" in perlipc, "IPC::SysV" documen- tation, and the "IPC::Shareable" module from CPAN. shutdown SOCKET,HOW Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the system call of the same name. shutdown(SOCKET, 0); # I/we have stopped reading data shutdown(SOCKET, 1); # I/we have stopped writing data shutdown(SOCKET, 2); # I/we have stopped using this socket This is useful with sockets when you want to tell the other side you're done writing but not done reading, or vice versa. It's also a more insistent form of close because it also dis- ables the file descriptor in any forked copies in other pro- cesses. sin EXPR sin Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of $_. For the inverse sine operation, you may use the "Math::Trig::asin" function, or use this relation: sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } sleep EXPR sleep Causes the script to sleep for EXPR seconds, or forever if no EXPR. May be interrupted if the process receives a signal such as "SIGALRM". Returns the number of seconds actually slept. You probably cannot mix "alarm" and "sleep" calls, because "sleep" is often implemented using "alarm". On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking sys- tem. For delays of finer granularity than one second, you may use Perl's "syscall" interface to access setitimer(2) if your sys- tem supports it, or else see "select" above. The Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the stan- dard distribution) may also help. See also the POSIX module's "pause" function. socket SOCKET,DOMAIN,TYPE,PROTOCOL Opens a socket of the specified kind and attaches it to file- handle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. You should "use Socket" first to get the proper definitions imported. See the examples in "Sockets: Client/Server Communication" in perlipc. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See "$^F" in perlvar. socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. If unimple- mented, yields a fatal error. Returns true if successful. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as deter- mined by the value of $^F. See "$^F" in perlvar. Some systems defined "pipe" in terms of "socketpair", in which a call to "pipe(Rdr, Wtr)" is essentially: use Socket; socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); shutdown(Rdr, 1); # no more writing for reader shutdown(Wtr, 0); # no more reading for writer See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair. sort SUBNAME LIST sort BLOCK LIST sort LIST In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of "sort()" is undefined. If SUBNAME or BLOCK is omitted, "sort"s in standard string com- parison order. If SUBNAME is specified, it gives the name of a subroutine that returns an integer less than, equal to, or greater than 0, depending on how the elements of the list are to be ordered. (The "<=>" and "cmp" operators are extremely useful in such routines.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine. If the subroutine's prototype is "($$)", the elements to be compared are passed by reference in @_, as for a normal subrou- tine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b (see example below). Note that in the latter case, it is usually counter-productive to declare $a and $b as lexicals. In either case, the subroutine may not be recursive. The val- ues to be compared are always passed by reference and should not be modified. You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with "goto". When "use locale" is in effect, "sort LIST" sorts LIST accord- ing to the current collation locale. See perllocale. sort() returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, mod- ifying an element of a list returned by sort() (for example, in a "foreach", "map" or "grep") actually modifies the element in the original list. This is usually something to be avoided when writing clear code. Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not stable, and could go quadratic. (A stable sort preserves the input order of elements that com- pare equal. Although quicksort's run time is O(NlogN) when averaged over all arrays of length N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort implementation was replaced with a stable mergesort algorithm whose worst-case behavior is O(NlogN). But benchmarks indi- cated that for some inputs, on some platforms, the original quicksort was faster. 5.8 has a sort pragma for limited con- trol of the sort. Its rather blunt control of the underlying algorithm may not persist into future Perls, but the ability to characterize the input or output in implementation independent ways quite probably will. See sort. Examples: # sort lexically @articles = sort @files; # same thing, but with explicit sort routine @articles = sort {$a cmp $b} @files; # now case-insensitively @articles = sort {uc($a) cmp uc($b)} @files; # same thing in reversed order @articles = sort {$b cmp $a} @files; # sort numerically ascending @articles = sort {$a <=> $b} @files; # sort numerically descending @articles = sort {$b <=> $a} @files; # this sorts the %age hash by value instead of key # using an in-line function @eldest = sort { $age{$b} <=> $age{$a} } keys %age; # sort using explicit subroutine name sub byage { $age{$a} <=> $age{$b}; # presuming numeric } @sortedclass = sort byage @class; sub backwards { $b cmp $a } @harry = qw(dog cat x Cain Abel); @george = qw(gone chased yz Punished Axed); print sort @harry; # prints AbelCaincatdogx print sort backwards @harry; # prints xdogcatCainAbel print sort @george, 'to', @harry; # prints AbelAxedCainPunishedcatchaseddoggonetoxyz # inefficiently sort by descending numeric compare using # the first integer after the first = sign, or the # whole record case-insensitively otherwise @new = sort { ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] || uc($a) cmp uc($b) } @old; # same thing, but much more efficiently; # we'll build auxiliary indices instead # for speed @nums = @caps = (); for (@old) { push @nums, /=(\d+)/; push @caps, uc($_); } @new = @old[ sort { $nums[$b] <=> $nums[$a] || $caps[$a] cmp $caps[$b] } 0..$#old ]; # same thing, but without any temps @new = map { $_->[0] } sort { $b->[1] <=> $a->[1] || $a->[2] cmp $b->[2] } map { [$_, /=(\d+)/, uc($_)] } @old; # using a prototype allows you to use any comparison subroutine # as a sort subroutine (including other package's subroutines) package other; sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here package main; @new = sort other::backwards @old; # guarantee stability, regardless of algorithm use sort 'stable'; @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; # force use of mergesort (not portable outside Perl 5.8) use sort '_mergesort'; # note discouraging _ @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; If you're using strict, you must not declare $a and $b as lexi- cals. They are package globals. That means if you're in the "main" package and type @articles = sort {$b <=> $a} @files; then $a and $b are $main::a and $main::b (or $::a and $::b), but if you're in the "FooPack" package, it's the same as typing @articles = sort {$FooPack::b <=> $FooPack::a} @files; The comparison function is required to behave. If it returns inconsistent results (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the results are not well-defined. Because "<=>" returns "undef" when either operand is "NaN" (not-a-number), and because "sort" will trigger a fatal error unless the result of a comparison is defined, when sorting with a comparison function like "$a <=> $b", be careful about lists that might contain a "NaN". The following example takes advan- tage of the fact that "NaN != NaN" to eliminate any "NaN"s from the input. @result = sort { $a <=> $b } grep { $_ == $_ } @input; splice ARRAY,OFFSET,LENGTH,LIST splice ARRAY,OFFSET,LENGTH splice ARRAY,OFFSET splice ARRAY Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. In list context, returns the elements removed from the array. In scalar context, returns the last element removed, or "undef" if no elements are removed. The array grows or shrinks as neces- sary. If OFFSET is negative then it starts that far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is negative, removes the ele- ments from OFFSET onward except for -LENGTH elements at the end of the array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of the array, perl issues a warning, and splices at the end of the array. The following equivalences hold (assuming "$[ == 0 and $#a >= $i" ) push(@a,$x,$y) splice(@a,@a,0,$x,$y) pop(@a) splice(@a,-1) shift(@a) splice(@a,0,1) unshift(@a,$x,$y) splice(@a,0,0,$x,$y) $a[$i] = $y splice(@a,$i,1,$y) Example, assuming array lengths are passed before arrays: sub aeq { # compare two list values my(@a) = splice(@_,0,shift); my(@b) = splice(@_,0,shift); return 0 unless @a == @b; # same len? while (@a) { return 0 if pop(@a) ne pop(@b); } return 1; } if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } split /PATTERN/,EXPR,LIMIT split /PATTERN/,EXPR split /PATTERN/ split Splits the string EXPR into a list of strings and returns that list. By default, empty leading fields are preserved, and empty trailing ones are deleted. (If all fields are empty, they are considered to be trailing.) In scalar context, returns the number of fields found and splits into the @_ array. Use of split in scalar context is deprecated, however, because it clobbers your subroutine argu- ments. If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace). Anything matching PATTERN is taken to be a delim- iter separating the fields. (Note that the delimiter may be longer than one character.) If LIMIT is specified and positive, it represents the maximum number of fields the EXPR will be split into, though the actual number of fields returned depends on the number of times PAT- TERN matches within EXPR. If LIMIT is unspecified or zero, trailing null fields are stripped (which potential users of "pop" would do well to remember). If LIMIT is negative, it is treated as if an arbitrarily large LIMIT had been specified. Note that splitting an EXPR that evaluates to the empty string always returns the empty list, regardless of the LIMIT speci- fied. A pattern matching the null string (not to be confused with a null pattern "//", which is just one member of the set of pat- terns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example: print join(':', split(/ */, 'hi there')); produces the output 'h:i:t:h:e:r:e'. As a special case for "split", using the empty pattern "//" specifically matches only the null string, and is not be con- fused with the regular use of "//" to mean "the last successful pattern match". So, for "split", the following: print join(':', split(//, 'hi there')); produces the output 'h:i: :t:h:e:r:e'. Empty leading (or trailing) fields are produced when there are positive width matches at the beginning (or end) of the string; a zero-width match at the beginning (or end) of the string does not produce an empty field. For example: print join(':', split(/(?=\w)/, 'hi there!')); produces the output 'h:i :t:h:e:r:e!'. The LIMIT parameter can be used to split a line partially ($login, $passwd, $remainder) = split(/:/, $_, 3); When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need. If the PATTERN contains parentheses, additional list elements are created from each matching substring in the delimiter. split(/([,-])/, "1-10,20", 3); produces the list value (1, '-', 10, ',', 20) If you had the entire header of a normal Unix email message in $header, you could split it up into fields and their values this way: $header =~ s/\n\s+/ /g; # fix continuation lines %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); The pattern "/PATTERN/" may be replaced with an expression to specify patterns that vary at runtime. (To do runtime compila- tion only once, use "/$variable/o".) As a special case, specifying a PATTERN of space (' ') will split on white space just as "split" with no arguments does. Thus, "split(' ')" can be used to emulate awk's default behav- ior, whereas "split(/ /)" will give you as many null initial fields as there are leading spaces. A "split" on "/\s+/" is like a "split(' ')" except that any leading whitespace produces a null first field. A "split" with no arguments really does a "split(' ', $_)" internally. A PATTERN of "/^/" is treated as if it were "/^/m", since it isn't much use otherwise. Example: open(PASSWD, '/etc/passwd'); while () { chomp; ($login, $passwd, $uid, $gid, $gcos, $home, $shell) = split(/:/); #... } As with regular pattern matching, any capturing parentheses that are not matched in a "split()" will be set to "undef" when returned: @fields = split /(A)|B/, "1A2B3"; # @fields is (1, 'A', 2, undef, 3) sprintf FORMAT, LIST Returns a string formatted by the usual "printf" conventions of the C library function "sprintf". See below for more details and see sprintf(3) or printf(3) on your system for an explana- tion of the general principles. For example: # Format number with up to 8 leading zeroes $result = sprintf("%08d", $number); # Round number to 3 digits after decimal point $rounded = sprintf("%.3f", $number); Perl does its own "sprintf" formatting--it emulates the C func- tion "sprintf", but it doesn't use it (except for floating- point numbers, and even then only the standard modifiers are allowed). As a result, any non-standard extensions in your local "sprintf" are not available from Perl. Unlike "printf", "sprintf" does not do what you probably mean when you pass it an array as your first argument. The array is given scalar context, and instead of using the 0th element of the array as the format, Perl will use the count of elements in the array as the format, which is almost never useful. Perl's "sprintf" permits the following universally-known con- versions: %% a percent sign %c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %o an unsigned integer, in octal %x an unsigned integer, in hexadecimal %e a floating-point number, in scientific notation %f a floating-point number, in fixed decimal notation %g a floating-point number, in %e or %f notation In addition, Perl permits the following widely-supported con- versions: %X like %x, but using upper-case letters %E like %e, but using an upper-case "E" %G like %g, but with an upper-case "E" (if applicable) %b an unsigned integer, in binary %p a pointer (outputs the Perl value's address in hexadecimal) %n special: *stores* the number of characters output so far into the next variable in the parameter list Finally, for backward (and we do mean "backward") compatibil- ity, Perl permits these unnecessary but widely-supported con- versions: %i a synonym for %d %D a synonym for %ld %U a synonym for %lu %O a synonym for %lo %F a synonym for %f Note that the number of exponent digits in the scientific nota- tion produced by %e, %E, %g and %G for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either "1.23e99" or "1.23e099". Between the "%" and the format letter, you may specify a number of additional attributes controlling the interpretation of the format. In order, these are: format parameter index An explicit format parameter index, such as "2$". By default sprintf will format the next unused argument in the list, but this allows you to take the arguments out of order, e.g.: printf '%2$d %1$d', 12, 34; # prints "34 12" printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" flags one or more of: space prefix positive number with a space + prefix positive number with a plus sign - left-justify within the field 0 use zeros, not spaces, to right-justify # prefix non-zero octal with "0", non-zero hex with "0x", non-zero binary with "0b" For example: printf '<% d>', 12; # prints "< 12>" printf '<%+d>', 12; # prints "<+12>" printf '<%6s>', 12; # prints "< 12>" printf '<%-6s>', 12; # prints "<12 >" printf '<%06s>', 12; # prints "<000012>" printf '<%#x>', 12; # prints "<0xc>" vector flag This flag tells perl to interpret the supplied string as a vector of integers, one for each character in the string. Perl applies the format to each integer in turn, then joins the resulting strings with a separator (a dot "." by default). This can be useful for displaying ordinal values of characters in arbitrary strings: printf "%vd", "AB\x{100}"; # prints "65.66.256" printf "version is v%vd\n", $^V; # Perl's version Put an asterisk "*" before the "v" to override the string to use to separate the numbers: printf "address is %*vX\n", ":", $addr; # IPv6 address printf "bits are %0*v8b\n", " ", $bits; # random bitstring You can also explicitly specify the argument number to use for the join string using e.g. "*2$v": printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses (minimum) width Arguments are usually formatted to be only as wide as required to display the given value. You can override the width by putting a number here, or get the width from the next argument (with "*") or from a specified argument (with e.g. "*2$"): printf '<%s>', "a"; # prints "" printf '<%6s>', "a"; # prints "< a>" printf '<%*s>', 6, "a"; # prints "< a>" printf '<%*2$s>', "a", 6; # prints "< a>" printf '<%2s>', "long"; # prints "" (does not truncate) If a field width obtained through "*" is negative, it has the same effect as the "-" flag: left-justification. precision, or maximum width You can specify a precision (for numeric conversions) or a maximum width (for string conversions) by specifying a "." followed by a number. For floating point formats, with the exception of 'g' and 'G', this specifies the number of dec- imal places to show (the default being 6), e.g.: # these examples are subject to system-specific variation printf '<%f>', 1; # prints "<1.000000>" printf '<%.1f>', 1; # prints "<1.0>" printf '<%.0f>', 1; # prints "<1>" printf '<%e>', 10; # prints "<1.000000e+01>" printf '<%.1e>', 10; # prints "<1.0e+01>" For 'g' and 'G', this specifies the maximum number of dig- its to show, including prior to the decimal point as well as after it, e.g.: # these examples are subject to system-specific variation printf '<%g>', 1; # prints "<1>" printf '<%.10g>', 1; # prints "<1>" printf '<%g>', 100; # prints "<100>" printf '<%.1g>', 100; # prints "<1e+02>" printf '<%.2g>', 100.01; # prints "<1e+02>" printf '<%.5g>', 100.01; # prints "<100.01>" printf '<%.4g>', 100.01; # prints "<100>" For integer conversions, specifying a precision implies that the output of the number itself should be zero-padded to this width: printf '<%.6x>', 1; # prints "<000001>" printf '<%#.6x>', 1; # prints "<0x000001>" printf '<%-10.6x>', 1; # prints "<000001 >" For string conversions, specifying a precision truncates the string to fit in the specified width: printf '<%.5s>', "truncated"; # prints "" printf '<%10.5s>', "truncated"; # prints "< trunc>" You can also get the precision from the next argument using ".*": printf '<%.6x>', 1; # prints "<000001>" printf '<%.*x>', 6, 1; # prints "<000001>" You cannot currently get the precision from a specified number, but it is intended that this will be possible in the future using e.g. ".*2$": printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>" size For numeric conversions, you can specify the size to inter- pret the number as using "l", "h", "V", "q", "L", or "ll". For integer conversions ("d u o x X b i D U O"), numbers are usually assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits), but you can override this to use instead one of the standard C types, as supported by the compiler used to build Perl: l interpret integer as C type "long" or "unsigned long" h interpret integer as C type "short" or "unsigned short" q, L or ll interpret integer as C type "long long", "unsigned long long". or "quads" (typically 64-bit integers) The last will produce errors if Perl does not understand "quads" in your installation. (This requires that either the platform natively supports quads or Perl was specifi- cally compiled to support quads.) You can find out whether your Perl supports quads via Config: use Config; ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) && print "quads\n"; For floating point conversions ("e f g E F G"), numbers are usually assumed to be the default floating point size on your platform (double or long double), but you can force 'long double' with "q", "L", or "ll" if your platform sup- ports them. You can find out whether your Perl supports long doubles via Config: use Config; $Config{d_longdbl} eq 'define' && print "long doubles\n"; You can find out whether Perl considers 'long double' to be the default floating point size to use on your platform via Config: use Config; ($Config{uselongdouble} eq 'define') && print "long doubles by default\n"; It can also be the case that long doubles and doubles are the same thing: use Config; ($Config{doublesize} == $Config{longdblsize}) && print "doubles are long doubles\n"; The size specifier "V" has no effect for Perl code, but it is supported for compatibility with XS code; it means 'use the standard size for a Perl integer (or floating-point number)', which is already the default for Perl code. order of arguments Normally, sprintf takes the next unused argument as the value to format for each format specification. If the for- mat specification uses "*" to require additional arguments, these are consumed from the argument list in the order in which they appear in the format specification before the value to format. Where an argument is specified using an explicit index, this does not affect the normal order for the arguments (even when the explicitly specified index would have been the next argument in any case). So: printf '<%*.*s>', $a, $b, $c; would use $a for the width, $b for the precision and $c as the value to format, while: print '<%*1$.*s>', $a, $b; would use $a for the width and the precision, and $b as the value to format. Here are some more examples - beware that when using an explicit index, the "$" may need to be escaped: printf "%2\$d %d\n", 12, 34; # will print "34 12\n" printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" If "use locale" is in effect, the character used for the deci- mal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale. sqrt EXPR sqrt Return the square root of EXPR. If EXPR is omitted, returns square root of $_. Only works on non-negative operands, unless you've loaded the standard Math::Complex module. use Math::Complex; print sqrt(-2); # prints 1.4142135623731i srand EXPR srand Sets the random number seed for the "rand" operator. The point of the function is to "seed" the "rand" function so that "rand" can produce a different sequence each time you run your program. If srand() is not called explicitly, it is called implicitly at the first use of the "rand" operator. However, this was not the case in versions of Perl before 5.004, so if your script will run under older Perl versions, it should call "srand". Most programs won't even call srand() at all, except those that need a cryptographically-strong starting point rather than the generally acceptable default, which is based on time of day, process ID, and memory allocation, or the /dev/urandom device, if available. You can call srand($seed) with the same $seed to reproduce the same sequence from rand(), but this is usually reserved for generating predictable results for testing or debugging. Oth- erwise, don't call srand() more than once in your program. Do not call srand() (i.e. without an argument) more than once in a script. The internal state of the random number generator should contain more entropy than can be provided by any seed, so calling srand() again actually loses randomness. Most implementations of "srand" take an integer and will silently truncate decimal numbers. This means "srand(42)" will usually produce the same results as "srand(42.1)". To be safe, always pass "srand" an integer. In versions of Perl prior to 5.004 the default seed was just the current "time". This isn't a particularly good seed, so many old programs supply their own seed value (often "time ^ $$" or "time ^ ($$ + ($$ << 15))"), but that isn't necessary any more. For cryptographic purposes, however, you need something much more random than the default seed. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example: srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); If you're particularly concerned with this, see the "Math::Tru- lyRandom" module in CPAN. Frequently called programs (like CGI scripts) that simply use time ^ $$ for a seed can fall prey to the mathematical property that a^b == (a+1)^(b+1) one-third of the time. So don't do that. stat FILEHANDLE stat EXPR stat Returns a 13-element list giving the status info for a file, either the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it stats $_. Returns a null list if the stat fails. Typically used as follows: ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, $atime,$mtime,$ctime,$blksize,$blocks) = stat($filename); Not all fields are supported on all filesystem types. Here are the meanings of the fields: 0 dev device number of filesystem 1 ino inode number 2 mode file mode (type and permissions) 3 nlink number of (hard) links to the file 4 uid numeric user ID of file's owner 5 gid numeric group ID of file's owner 6 rdev the device identifier (special files only) 7 size total size of file, in bytes 8 atime last access time in seconds since the epoch 9 mtime last modify time in seconds since the epoch 10 ctime inode change time in seconds since the epoch (*) 11 blksize preferred block size for file system I/O 12 blocks actual number of blocks allocated (The epoch was at 00:00 January 1, 1970 GMT.) (*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-portable. In particular, you cannot expect it to be a "creation time", see "Files and Filesystems" in perlport for details. If "stat" is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last "stat", "lstat", or filetest are returned. Example: if (-x $file && (($d) = stat(_)) && $d < 0) { print "$file is executable NFS file\n"; } (This works on machines only for which the device number is negative under NFS.) Because the mode contains both the file type and its permis- sions, you should mask off the file type portion and (s)printf using a "%o" if you want to see the real permissions. $mode = (stat($filename))[2]; printf "Permissions are %04o\n", $mode & 07777; In scalar context, "stat" returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle "_". The File::stat module provides a convenient, by-name access mechanism: use File::stat; $sb = stat($filename); printf "File is %s, size is %s, perm %04o, mtime %s\n", $filename, $sb->size, $sb->mode & 07777, scalar localtime $sb->mtime; You can import symbolic mode constants ("S_IF*") and functions ("S_IS*") from the Fcntl module: use Fcntl ':mode'; $mode = (stat($filename))[2]; $user_rwx = ($mode & S_IRWXU) >> 6; $group_read = ($mode & S_IRGRP) >> 3; $other_execute = $mode & S_IXOTH; printf "Permissions are %04o\n", S_IMODE($mode), "\n"; $is_setuid = $mode & S_ISUID; $is_setgid = S_ISDIR($mode); You could write the last two using the "-u" and "-d" operators. The commonly available "S_IF*" constants are # Permissions: read, write, execute, for user, group, others. S_IRWXU S_IRUSR S_IWUSR S_IXUSR S_IRWXG S_IRGRP S_IWGRP S_IXGRP S_IRWXO S_IROTH S_IWOTH S_IXOTH # Setuid/Setgid/Stickiness/SaveText. # Note that the exact meaning of these is system dependent. S_ISUID S_ISGID S_ISVTX S_ISTXT # File types. Not necessarily all are available on your system. S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. S_IREAD S_IWRITE S_IEXEC and the "S_IF*" functions are S_IMODE($mode) the part of $mode containing the permission bits and the setuid/setgid/sticky bits S_IFMT($mode) the part of $mode containing the file type which can be bit-anded with e.g. S_IFREG or with the following functions # The operators -f, -d, -l, -b, -c, -p, and -S. S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) # No direct -X operator counterpart, but for the first one # the -g operator is often equivalent. The ENFMT stands for # record flocking enforcement, a platform-dependent feature. S_ISENFMT($mode) S_ISWHT($mode) See your native chmod(2) and stat(2) documentation for more details about the "S_*" constants. To get status info for a symbolic link instead of the target file behind the link, use the "lstat" function. study SCALAR study Takes extra time to study SCALAR ($_ if unspecified) in antici- pation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribution of character frequencies in the string to be searched--you probably want to compare run times with and with- out it to see which runs faster. Those loops that scan for many short constant strings (including the constant parts of more complex patterns) will benefit most. You may have only one "study" active at a time--if you study a different scalar the first is "unstudied". (The way "study" works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the 'k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this "rarest" character are examined.) For example, here is a loop that inserts index producing entries before any line containing a certain pattern: while (<>) { study; print ".IX foo\n" if /\bfoo\b/; print ".IX bar\n" if /\bbar\b/; print ".IX blurfl\n" if /\bblurfl\b/; # ... print; } In searching for "/\bfoo\b/", only those locations in $_ that contain "f" will be looked at, because "f" is rarer than "o". In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place. Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and "eval" that to avoid recompiling all your patterns all the time. Together with undefining $/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep(1). The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a match: $search = 'while (<>) { study;'; foreach $word (@words) { $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; } $search .= "}"; @ARGV = @files; undef $/; eval $search; # this screams $/ = "\n"; # put back to normal input delimiter foreach $file (sort keys(%seen)) { print $file, "\n"; } sub NAME BLOCK sub NAME (PROTO) BLOCK sub NAME : ATTRS BLOCK sub NAME (PROTO) : ATTRS BLOCK This is subroutine definition, not a real function per se. Without a BLOCK it's just a forward declaration. Without a NAME, it's an anonymous function declaration, and does actually return a value: the CODE ref of the closure you just created. See perlsub and perlref for details about subroutines and ref- erences, and attributes and Attribute::Handlers for more infor- mation about attributes. substr EXPR,OFFSET,LENGTH,REPLACEMENT substr EXPR,OFFSET,LENGTH substr EXPR,OFFSET Extracts a substring out of EXPR and returns it. First charac- ter is at offset 0, or whatever you've set $[ to (but don't do that). If OFFSET is negative (or more precisely, less than $[), starts that far from the end of the string. If LENGTH is omitted, returns everything to the end of the string. If LENGTH is negative, leaves that many characters off the end of the string. You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue. If you assign something shorter than LENGTH, the string will shrink, and if you assign some- thing longer than LENGTH, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using "sprintf". If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within the string is returned. If the substring is beyond either end of the string, substr() returns the undefined value and produces a warning. When used as an lvalue, specifying a substring that is entirely outside the string is a fatal error. Here's an example showing the behavior for boundary cases: my $name = 'fred'; substr($name, 4) = 'dy'; # $name is now 'freddy' my $null = substr $name, 6, 2; # returns '' (no warning) my $oops = substr $name, 7; # returns undef, with warning substr($name, 7) = 'gap'; # fatal error An alternative to using substr() as an lvalue is to specify the replacement string as the 4th argument. This allows you to replace parts of the EXPR and return what was there before in one operation, just as you can with splice(). symlink OLDFILE,NEWFILE Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't sup- port symbolic links, produces a fatal error at run time. To check for that, use eval: $symlink_exists = eval { symlink("",""); 1 }; syscall NUMBER, LIST Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, produces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. You can't use a string literal (or other read-only string) as an argument to "syscall" because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. This emu- lates the "syswrite" function (or vice versa): require 'syscall.ph'; # may need to run h2ph $s = "hi there\n"; syscall(&SYS_write, fileno(STDOUT), $s, length $s); Note that Perl supports passing of up to only 14 arguments to your system call, which in practice should usually suffice. Syscall returns whatever value returned by the system call it calls. If the system call fails, "syscall" returns "-1" and sets $! (errno). Note that some system calls can legitimately return "-1". The proper way to handle such calls is to assign "$!=0;" before the call and check the value of $! if syscall returns "-1". There's a problem with "syscall(&SYS_pipe)": it returns the file number of the read end of the pipe it creates. There is no way to retrieve the file number of the other end. You can avoid this problem by using "pipe" instead. sysopen FILEHANDLE,FILENAME,MODE sysopen FILEHANDLE,FILENAME,MODE,PERMS Opens the file whose filename is given by FILENAME, and asso- ciates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This function calls the underlying operating system's "open" func- tion with the parameters FILENAME, MODE, PERMS. The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module "Fcntl". See the documentation of your operating system's "open" to see which values and flag bits are available. You may combine several flags using the "|"-operator. Some of the most common values are "O_RDONLY" for opening the file in read-only mode, "O_WRONLY" for opening the file in write-only mode, and "O_RDWR" for opening the file in read- write mode. For historical reasons, some values work on almost every system supported by perl: zero means read-only, one means write-only, and two means read/write. We know that these values do not work under OS/390 & VM/ESA Unix and on the Macintosh; you prob- ably don't want to use them in new code. If the file named by FILENAME does not exist and the "open" call creates it (typically because MODE includes the "O_CREAT" flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to "sysopen", Perl uses the octal value 0666. These permission values need to be in octal, and are modified by your process's current "umask". In many systems the "O_EXCL" flag is available for opening files in exclusive mode. This is not locking: exclusiveness means here that if the file already exists, sysopen() fails. "O_EXCL" may not work on network filesystems, and has no effect unless the "O_CREAT" flag is set as well. Setting "O_CREAT|O_EXCL" prevents the file from being opened if it is a symbolic link. It does not protect against symbolic links in the file's path. Sometimes you may want to truncate an already-existing file. This can be done using the "O_TRUNC" flag. The behavior of "O_TRUNC" with "O_RDONLY" is undefined. You should seldom if ever use 0644 as argument to "sysopen", because that takes away the user's option to have a more per- missive umask. Better to omit it. See the perlfunc(1) entry on "umask" for more on this. Note that "sysopen" depends on the fdopen() C library function. On many UNIX systems, fdopen() is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider rebuilding Perl to use the "sfio" library, or perhaps using the POSIX::open() function. See perlopentut for a kinder, gentler explanation of opening files. sysread FILEHANDLE,SCALAR,LENGTH,OFFSET sysread FILEHANDLE,SCALAR,LENGTH Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the system call read(2). It bypasses buffered IO, so mixing this with other kinds of reads, "print", "write", "seek", "tell", or "eof" can cause confusion because the perlio or stdio layers usually buffers data. Returns the number of bytes actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read. An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended. There is no syseof() function, which is ok, since eof() doesn't work very well on device files (like ttys) anyway. Use sys- read() and check for a return value for 0 to decide whether you're done. Note that if the filehandle has been marked as ":utf8" Unicode characters are read instead of bytes (the LENGTH, OFFSET, and the return value of sysread() are in Unicode characters). The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode", "open", and the "open" pragma, open. sysseek FILEHANDLE,POSITION,WHENCE Sets FILEHANDLE's system position in bytes using the system call lseek(2). FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to POSITION, 1 to set the it to the current position plus POSITION, and 2 to set it to EOF plus POSITION (typically negative). Note the in bytes: even if the filehandle has been set to oper- ate on characters (for example by using the ":utf8" I/O layer), tell() will return byte offsets, not character offsets (because implementing that would render sysseek() very slow). sysseek() bypasses normal buffered IO, so mixing this with reads (other than "sysread", for example "<>" or read()) "print", "write", "seek", "tell", or "eof" may cause confusion. For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the file, current posi- tion, end of the file) from the Fcntl module. Use of the con- stants is also more portable than relying on 0, 1, and 2. For example to define a "systell" function: use Fcntl 'SEEK_CUR'; sub systell { sysseek($_[0], 0, SEEK_CUR) } Returns the new position, or the undefined value on failure. A position of zero is returned as the string "0 but true"; thus "sysseek" returns true on success and false on failure, yet you can still easily determine the new position. system LIST system PROGRAM LIST Does exactly the same thing as "exec LIST", except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. If there is more than one argument in LIST, or if LIST is an array with more than one value, starts the program given by the first element of the list with arguments given by the rest of the list. If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to "execvp", which is more efficient. Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles. The return value is the exit status of the program as returned by the "wait" call. To get the actual exit value, shift right by eight (see below). See also "exec". This is not what you want to use to capture the output from a command, for that you should use merely backticks or "qx//", as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason). Like "exec", "system" allows you to lie to a program about its name if you use the "system PROGRAM LIST" syntax. Again, see "exec". Since "SIGINT" and "SIGQUIT" are ignored during the execution of "system", if you expect your program to terminate on receipt of these signals you will need to arrange to do so yourself based on the return value. @args = ("command", "arg1", "arg2"); system(@args) == 0 or die "system @args failed: $?" You can check all the failure possibilities by inspecting $? like this: if ($? == -1) { print "failed to execute: $!\n"; } elsif ($? & 127) { printf "child died with signal %d, %s coredump\n", ($? & 127), ($? & 128) ? 'with' : 'without'; } else { printf "child exited with value %d\n", $? >> 8; } or more portably by using the W*() calls of the POSIX exten- sion; see perlport for more information. When the arguments get executed via the system shell, results and return codes will be subject to its quirks and capabili- ties. See "`STRING`" in perlop and "exec" for details. syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET syswrite FILEHANDLE,SCALAR,LENGTH syswrite FILEHANDLE,SCALAR Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses buffered IO, so mixing this with reads (other than sysread()), "print", "write", "seek", "tell", or "eof" may cause confusion because the perlio and stdio layers usually buffers data. Returns the number of bytes actually written, or "undef" if there was an error (in this case the errno variable $! is also set). If the LENGTH is greater than the available data in the SCALAR after the OFFSET, only as much data as is available will be written. An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET speci- fies writing that many characters counting backwards from the end of the string. In the case the SCALAR is empty you can use OFFSET but only zero offset. Note that if the filehandle has been marked as ":utf8", Unicode characters are written instead of bytes (the LENGTH, OFFSET, and the return value of syswrite() are in UTF-8 encoded Unicode characters). The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode", "open", and the "open" pragma, open. tell FILEHANDLE tell Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read. Note the in bytes: even if the filehandle has been set to oper- ate on characters (for example by using the ":utf8" open layer), tell() will return byte offsets, not character offsets (because that would render seek() and tell() rather slow). The return value of tell() for the standard streams like the STDIN depends on the operating system: it may return -1 or something else. tell() on pipes, fifos, and sockets usually returns -1. There is no "systell" function. Use "sysseek(FH, 0, 1)" for that. Do not use tell() (or other buffered I/O operations) on a file handle that has been manipulated by sysread(), syswrite() or sysseek(). Those functions ignore the buffering, while tell() does not. telldir DIRHANDLE Returns the current position of the "readdir" routines on DIRHANDLE. Value may be given to "seekdir" to access a partic- ular location in a directory. "telldir" has the same caveats about possible directory compaction as the corresponding system library routine. tie VARIABLE,CLASSNAME,LIST This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the "new" method of the class (meaning "TIESCALAR", "TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically these are arguments such as might be passed to the "dbm_open()" function of C. The object returned by the "new" method is also returned by the "tie" function, which would be useful if you want to access other methods in CLASSNAME. Note that functions such as "keys" and "values" may return huge lists when used on large objects, like DBM files. You may pre- fer to use the "each" function to iterate over such. Example: # print out history file offsets use NDBM_File; tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); while (($key,$val) = each %HIST) { print $key, ' = ', unpack('L',$val), "\n"; } untie(%HIST); A class implementing a hash should have the following methods: TIEHASH classname, LIST FETCH this, key STORE this, key, value DELETE this, key CLEAR this EXISTS this, key FIRSTKEY this NEXTKEY this, lastkey SCALAR this DESTROY this UNTIE this A class implementing an ordinary array should have the follow- ing methods: TIEARRAY classname, LIST FETCH this, key STORE this, key, value FETCHSIZE this STORESIZE this, count CLEAR this PUSH this, LIST POP this SHIFT this UNSHIFT this, LIST SPLICE this, offset, length, LIST EXTEND this, count DESTROY this UNTIE this A class implementing a file handle should have the following methods: TIEHANDLE classname, LIST READ this, scalar, length, offset READLINE this GETC this WRITE this, scalar, length, offset PRINT this, LIST PRINTF this, format, LIST BINMODE this EOF this FILENO this SEEK this, position, whence TELL this OPEN this, mode, LIST CLOSE this DESTROY this UNTIE this A class implementing a scalar should have the following meth- ods: TIESCALAR classname, LIST FETCH this, STORE this, value DESTROY this UNTIE this Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle. Unlike "dbmopen", the "tie" function will not use or require a module for you--you need to do that explicitly yourself. See DB_File or the Config module for interesting "tie" implementa- tions. For further details see perltie, "tied VARIABLE". tied VARIABLE Returns a reference to the object underlying VARIABLE (the same value that was originally returned by the "tie" call that bound the variable to a package.) Returns the undefined value if VARIABLE isn't tied to a package. time Returns the number of non-leap seconds since whatever time the system considers to be the epoch, suitable for feeding to "gmtime" and "localtime". On most systems the epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Clas- sic which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch. For measuring time in better granularity than one second, you may use either the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution), or if you have gettimeofday(2), you may be able to use the "syscall" interface of Perl. See perlfaq8 for details. times Returns a four-element list giving the user and system times, in seconds, for this process and the children of this process. ($user,$system,$cuser,$csystem) = times; In scalar context, "times" returns $user. tr/// The transliteration operator. Same as "y///". See perlop. truncate FILEHANDLE,LENGTH truncate EXPR,LENGTH Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system. Returns true if successful, the undefined value otherwise. The behavior is undefined if LENGTH is greater than the length of the file. uc EXPR uc Returns an uppercased version of EXPR. This is the internal function implementing the "\U" escape in double-quoted strings. Respects current LC_CTYPE locale if "use locale" in force. See perllocale and perlunicode for more details about locale and Unicode support. It does not attempt to do titlecase mapping on initial letters. See "ucfirst" for that. If EXPR is omitted, uses $_. ucfirst EXPR ucfirst Returns the value of EXPR with the first character in uppercase (titlecase in Unicode). This is the internal function imple- menting the "\u" escape in double-quoted strings. Respects current LC_CTYPE locale if "use locale" in force. See perllo- cale and perlunicode for more details about locale and Unicode support. If EXPR is omitted, uses $_. umask EXPR umask Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask. The Unix permission "rwxr-x---" is represented as three sets of three bits, or three octal digits: 0750 (the leading 0 indi- cates octal and isn't one of the digits). The "umask" value is such a number representing disabled permissions bits. The per- mission (or "mode") values you pass "mkdir" or "sysopen" are modified by your umask, so even if you tell "sysopen" to create a file with permissions 0777, if your umask is 0022 then the file will actually be created with permissions 0755. If your "umask" were 0027 (group can't write; others can't read, write, or execute), then passing "sysopen" 0666 would create a file with mode 0640 ("0666 &~ 027" is 0640). Here's some advice: supply a creation mode of 0666 for regular files (in "sysopen") and one of 0777 for directories (in "mkdir") and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions bet- ter left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on. If umask(2) is not implemented on your system and you are try- ing to restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a fatal error at run time. If umask(2) is not imple- mented and you are not trying to restrict access for yourself, returns "undef". Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also "oct", if all you have is a string. undef EXPR undef Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array (using "@"), a hash (using "%"), a subroutine (using "&"), or a typeglob (using "*"). (Saying "undef $hash{$key}" will probably not do what you expect on most predefined variables or DBM list values, so don't do that; see delete.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable or pass as a parameter. Exam- ples: undef $foo; undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; undef @ary; undef %hash; undef &mysub; undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; select undef, undef, undef, 0.25; ($a, $b, undef, $c) = &foo; # Ignore third value returned Note that this is a unary operator, not a list operator. unlink LIST unlink Deletes a list of files. Returns the number of files success- fully deleted. $cnt = unlink 'a', 'b', 'c'; unlink @goners; unlink <*.bak>; Note: "unlink" will not attempt to delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Finally, using "unlink" on directories is not supported on many operating systems. Use "rmdir" instead. If LIST is omitted, uses $_. unpack TEMPLATE,EXPR "unpack" does the reverse of "pack": it takes a string and expands it out into a list of values. (In scalar context, it returns merely the first value produced.) The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately to a value. Typically, either the string is a result of "pack", or the bytes of the string represent a C structure of some kind. The TEMPLATE has the same format as in the "pack" function. Here's a subroutine that does substring: sub substr { my($what,$where,$howmuch) = @_; unpack("x$where a$howmuch", $what); } and then there's sub ordinal { unpack("c",$_[0]); } # same as ord() In addition to fields allowed in pack(), you may prefix a field with a % to indicate that you want a -bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. Checksum is calculated by summing numeric values of expanded values (for string fields the sum of "ord($char)" is taken, for bit fields the sum of zeroes and ones). For example, the following computes the same number as the Sys- tem V sum program: $checksum = do { local $/; # slurp! unpack("%32C*",<>) % 65535; }; The following efficiently counts the number of set bits in a bit vector: $setbits = unpack("%32b*", $selectmask); The "p" and "P" formats should be used with care. Since Perl has no way of checking whether the value passed to "unpack()" corresponds to a valid memory location, passing a pointer value that's not known to be valid is likely to have disastrous con- sequences. If there are more pack codes or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: in some cases, the repeat count is decreased, or "unpack()" will produce null strings or zeroes, or terminate with an error. If the input string is longer than one described by the TEMPLATE, the rest is ignored. See "pack" for more examples and notes. untie VARIABLE Breaks the binding between a variable and a package. (See "tie".) Has no effect if the variable is not tied. unshift ARRAY,LIST Does the opposite of a "shift". Or the opposite of a "push", depending on how you look at it. Prepends list to the front of the array, and returns the new number of elements in the array. unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. Use "reverse" to do the reverse. use Module VERSION LIST use Module VERSION use Module LIST use Module use VERSION Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to BEGIN { require Module; import Module LIST; } except that Module must be a bareword. VERSION may be either a numeric argument such as 5.006, which will be compared to $], or a literal of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION. A fatal error is produced if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with "require", which can do a similar check at run time. Specifying VERSION as a literal of the form v5.6.1 should gen- erally be avoided, because it leads to misleading error mes- sages under earlier versions of Perl that do not support this syntax. The equivalent numeric version should be used instead. use v5.6.1; # compile time version check use 5.6.1; # ditto use 5.006_001; # ditto; preferred for backwards compatibility This is often useful if you need to check the current Perl ver- sion before "use"ing library modules that have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.) The "BEGIN" forces the "require" and "import" to happen at com- pile time. The "require" makes sure the module is loaded into memory if it hasn't been yet. The "import" is not a builtin--it's just an ordinary static method call into the "Module" package to tell the module to import the list of fea- tures back into the current package. The module can implement its "import" method any way it likes, though most modules just choose to derive their "import" method via inheritance from the "Exporter" class that is defined in the "Exporter" module. See Exporter. If no "import" method can be found then the call is skipped. If you do not want to call the package's "import" method (for instance, to stop your namespace from being altered), explic- itly supply the empty list: use Module (); That is exactly equivalent to BEGIN { require Module } If the VERSION argument is present between Module and LIST, then the "use" will call the VERSION method in class Module with the given version as an argument. The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VER- SION. Again, there is a distinction between omitting LIST ("import" called with no arguments) and an explicit empty LIST "()" ("import" not called). Note that there is no comma after VER- SION! Because this is a wide-open interface, pragmas (compiler direc- tives) are also implemented this way. Currently implemented pragmas are: use constant; use diagnostics; use integer; use sigtrap qw(SEGV BUS); use strict qw(subs vars refs); use subs qw(afunc blurfl); use warnings qw(all); use sort qw(stable _quicksort _mergesort); Some of these pseudo-modules import semantics into the current block scope (like "strict" or "integer", unlike ordinary mod- ules, which import symbols into the current package (which are effective through the end of the file). There's a corresponding "no" command that unimports meanings imported by "use", i.e., it calls "unimport Module LIST" instead of "import". no integer; no strict 'refs'; no warnings; See perlmodlib for a list of standard modules and pragmas. See perlrun for the "-M" and "-m" command-line options to perl that give "use" functionality from the command-line. utime LIST Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modification times, in that order. Returns the number of files successfully changed. The inode change time of each file is set to the current time. For exam- ple, this code has the same effect as the Unix touch(1) command when the files already exist and belong to the user running the program: #!/usr/bin/perl $atime = $mtime = time; utime $atime, $mtime, @ARGV; Since perl 5.7.2, if the first two elements of the list are "undef", then the utime(2) function in the C library will be called with a null second argument. On most systems, this will set the file's access and modification times to the current time (i.e. equivalent to the example above) and will even work on other users' files where you have write permission: utime undef, undef, @ARGV; Under NFS this will use the time of the NFS server, not the time of the local machine. If there is a time synchronization problem, the NFS server and local machine will have different times. The Unix touch(1) command will in fact normally use this form instead of the one shown in the first example. Note that only passing one of the first two elements as "undef" will be equivalent of passing it as 0 and will not have the same effect as described when they are both "undef". This case will also trigger an uninitialized warning. values HASH Returns a list consisting of all the values of the named hash. (In a scalar context, returns the number of values.) The values are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be the same order as either the "keys" or "each" function would produce on the same (unmodi- fied) hash. Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see "Algo- rithmic Complexity Attacks" in perlsec). As a side effect, calling values() resets the HASH's internal iterator, see "each". (In particular, calling values() in void context resets the iterator with no other overhead.) Note that the values are not copied, which means modifying them will modify the contents of the hash: for (values %hash) { s/foo/bar/g } # modifies %hash values for (@hash{keys %hash}) { s/foo/bar/g } # same See also "keys", "each", and "sort". vec EXPR,OFFSET,BITS Treats the string in EXPR as a bit vector made up of elements of width BITS, and returns the value of the element specified by OFFSET as an unsigned integer. BITS therefore specifies the number of bits that are reserved for each element in the bit vector. This must be a power of two from 1 to 32 (or 64, if your platform supports that). If BITS is 8, "elements" coincide with bytes of the input string. If BITS is 16 or more, bytes of the input string are grouped into chunks of size BITS/8, and each group is converted to a number as with pack()/unpack() with big-endian formats "n"/"N" (and analogously for BITS==64). See "pack" for details. If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into 8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the single input byte "chr(0x36)" into two groups gives a list "(0x6, 0x3)"; breaking it into 4 groups gives "(0x2, 0x1, 0x3, 0x0)". "vec" may also be assigned to, in which case parentheses are needed to give the expression the correct precedence as in vec($image, $max_x * $x + $y, 8) = 3; If the selected element is outside the string, the value 0 is returned. If an element off the end of the string is written to, Perl will first extend the string with sufficiently many zero bytes. It is an error to try to write off the beginning of the string (i.e. negative OFFSET). The string should not contain any character with the value > 255 (which can only happen if you're using UTF-8 encoding). If it does, it will be treated as something that is not UTF-8 encoded. When the "vec" was assigned to, other parts of your program will also no longer consider the string to be UTF-8 encoded. In other words, if you do have such characters in your string, vec() will operate on the actual byte string, and not the conceptual character string. Strings created with "vec" can also be manipulated with the logical operators "|", "&", "^", and "~". These operators will assume a bit vector operation is desired when both operands are strings. See "Bitwise String Operators" in perlop. The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the string after each step. Note that this code works in the same way on big-endian or lit- tle-endian machines. my $foo = ''; vec($foo, 0, 32) = 0x5065726C; # 'Perl' # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') vec($foo, 2, 16) = 0x5065; # 'PerlPe' vec($foo, 3, 16) = 0x726C; # 'PerlPerl' vec($foo, 8, 8) = 0x50; # 'PerlPerlP' vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72" vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c" To transform a bit vector into a string or list of 0's and 1's, use these: $bits = unpack("b*", $vector); @bits = split(//, unpack("b*", $vector)); If you know the exact length in bits, it can be used in place of the "*". Here is an example to illustrate how the bits actually fall in place: #!/usr/bin/perl -wl print <<'EOT'; 0 1 2 3 unpack("V",$_) 01234567890123456789012345678901 ------------------------------------------------------------------ EOT for $w (0..3) { $width = 2**$w; for ($shift=0; $shift < $width; ++$shift) { for ($off=0; $off < 32/$width; ++$off) { $str = pack("B*", "0"x32); $bits = (1<<$shift); vec($str, $off, $width) = $bits; $res = unpack("b*",$str); $val = unpack("V", $str); write; } } } format STDOUT = vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> $off, $width, $bits, $val, $res . __END__ Regardless of the machine architecture on which it is run, the above example should print the following table: 0 1 2 3 unpack("V",$_) 01234567890123456789012345678901 ------------------------------------------------------------------ vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 wait Behaves like the wait(2) system call on your system: it waits for a child process to terminate and returns the pid of the deceased process, or "-1" if there are no child processes. The status is returned in $?. Note that a return value of "-1" could mean that child processes are being automatically reaped, as described in perlipc. waitpid PID,FLAGS Waits for a particular child process to terminate and returns the pid of the deceased process, or "-1" if there is no such child process. On some systems, a value of 0 indicates that there are processes still running. The status is returned in $?. If you say use POSIX ":sys_wait_h"; #... do { $kid = waitpid(-1, WNOHANG); } until $kid > 0; then you can do a non-blocking wait for all pending zombie pro- cesses. Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) system calls. However, wait- ing for a particular pid with FLAGS of 0 is implemented every- where. (Perl emulates the system call by remembering the sta- tus values of processes that have exited but have not been har- vested by the Perl script yet.) Note that on some systems, a return value of "-1" could mean that child processes are being automatically reaped. See per- lipc for details, and for other examples. wantarray Returns true if the context of the currently executing subrou- tine or "eval" is looking for a list value. Returns false if the context is looking for a scalar. Returns the undefined value if the context is looking for no value (void context). return unless defined wantarray; # don't bother doing more my @a = complex_calculation(); return wantarray ? @a : "@a"; "wantarray()"'s result is unspecified in the top level of a file, in a "BEGIN", "CHECK", "INIT" or "END" block, or in a "DESTROY" method. This function should have been named wantlist() instead. warn LIST Produces a message on STDERR just like "die", but doesn't exit or throw an exception. If LIST is empty and $@ already contains a value (typically from a previous eval) that value is used after appending "\t...caught" to $@. This is useful for staying almost, but not entirely similar to "die". If $@ is empty then the string "Warning: Something's wrong" is used. No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's responsibility to deal with the message as it sees fit (like, for instance, converting it into a "die"). Most handlers must therefore make arrangements to actually display the warnings that they are not prepared to deal with, by calling "warn" again in the handler. Note that this is quite safe and will not produce an endless loop, since "__WARN__" hooks are not called from inside one. You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call "die" again to change it). Using a "__WARN__" handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example: # wipe out *all* compile-time warnings BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } my $foo = 10; my $foo = 20; # no warning about duplicate my $foo, # but hey, you asked for it! # no compile-time or run-time warnings before here $DOWARN = 1; # run-time warnings enabled after here warn "\$foo is alive and $foo!"; # does show up See perlvar for details on setting %SIG entries, and for more examples. See the Carp module for other kinds of warnings using its carp() and cluck() functions. write FILEHANDLE write EXPR write Writes a formatted record (possibly multi-line) to the speci- fied FILEHANDLE, using the format associated with that file. By default the format for a file is the one having the same name as the filehandle, but the format for the current output channel (see the "select" function) may be set explicitly by assigning the name of the format to the $~ variable. Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed, a special top-of- page format is used to format the new page header, and then the record is written. By default the top-of-page format is the name of the filehandle with "_TOP" appended, but it may be dynamically set to the format of your choice by assigning the name to the $^ variable while the filehandle is selected. The number of lines remaining on the current page is in variable "$-", which can be set to 0 to force a new page. If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as STDOUT but may be changed by the "select" operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see perlform. Note that write is not the opposite of "read". Unfortunately. y/// The transliteration operator. Same as "tr///". See perlop. perl v5.8.8 2006-01-07 PERLFUNC(1)
=11704
+165
(75)