) {... 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)