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tclvars(n) Tcl Built-In Commands tclvars(n)
NAME
tclvars - Variables used by Tcl
DESCRIPTION
The following global variables are created and managed automatically by
the Tcl library. Except where noted below, these variables should nor-
mally be treated as read-only by application-specific code and by
users.
env This variable is maintained by Tcl as an array whose elements
are the environment variables for the process. Reading an ele-
ment will return the value of the corresponding environment
variable. Setting an element of the array will modify the cor-
responding environment variable or create a new one if it
doesn't already exist. Unsetting an element of env will remove
the corresponding environment variable. Changes to the env
array will affect the environment passed to children by commands
like exec. If the entire env array is unset then Tcl will stop
monitoring env accesses and will not update environment vari-
ables.
Under Windows, the environment variables PATH and COMSPEC in any
capitalization are converted automatically to upper case. For
instance, the PATH variable could be exported by the operating
system as ``path'', ``Path'', ``PaTh'', etc., causing otherwise
simple Tcl code to have to support many special cases. All
other environment variables inherited by Tcl are left unmodi-
fied. Setting an env array variable to blank is the same as
unsetting it as this is the behavior of the underlying Windows
OS. It should be noted that relying on an existing and empty
environment variable won't work on windows and is discouraged
for cross-platform usage.
On the Macintosh, the environment variable is constructed by Tcl
as no global environment variable exists. The environment vari-
ables that are created for Tcl include:
LOGIN This holds the Chooser name of the Macintosh.
USER This also holds the Chooser name of the Macintosh.
SYS_FOLDER
The path to the system directory.
APPLE_M_FOLDER
The path to the Apple Menu directory.
CP_FOLDER
The path to the control panels directory.
DESK_FOLDER
The path to the desk top directory.
EXT_FOLDER
The path to the system extensions directory.
PREF_FOLDER
The path to the preferences directory.
PRINT_MON_FOLDER
The path to the print monitor directory.
SHARED_TRASH_FOLDER
The path to the network trash directory.
TRASH_FOLDER
The path to the trash directory.
START_UP_FOLDER
The path to the start up directory.
HOME The path to the application's default directory.
You can also create your own environment variables for the Mac-
intosh. A file named Tcl Environment Variables may be placed
in the preferences folder in the Mac system folder. Each line
of this file should be of the form VAR_NAME=var_data.
The last alternative is to place environment variables in a
'STR#' resource named Tcl Environment Variables of the applica-
tion. This is considered a little more ``Mac like'' than a Unix
style Environment Variable file. Each entry in the 'STR#'
resource has the same format as above. The source code file
tclMacEnv.c contains the implementation of the env mechanisms.
This file contains many #define's that allow customization of
the env mechanisms to fit your applications needs.
errorCode
After an error has occurred, this variable will be set to hold
additional information about the error in a form that is easy to
process with programs. errorCode consists of a Tcl list with
one or more elements. The first element of the list identifies
a general class of errors, and determines the format of the rest
of the list. The following formats for errorCode are used by
the Tcl core; individual applications may define additional for-
mats.
ARITH code msg
This format is used when an arithmetic error occurs (e.g.
an attempt to divide by zero in the expr command). Code
identifies the precise error and msg provides a human-
readable description of the error. Code will be either
DIVZERO (for an attempt to divide by zero), DOMAIN (if an
argument is outside the domain of a function, such as
acos(-3)), IOVERFLOW (for integer overflow), OVERFLOW
(for a floating-point overflow), or UNKNOWN (if the cause
of the error cannot be determined).
CHILDKILLED pid sigName msg
This format is used when a child process has been killed
because of a signal. The second element of errorCode
will be the process's identifier (in decimal). The third
element will be the symbolic name of the signal that
caused the process to terminate; it will be one of the
names from the include file signal.h, such as SIGPIPE.
The fourth element will be a short human-readable message
describing the signal, such as ``write on pipe with no
readers'' for SIGPIPE.
CHILDSTATUS pid code
This format is used when a child process has exited with
a non-zero exit status. The second element of errorCode
will be the process's identifier (in decimal) and the
third element will be the exit code returned by the
process (also in decimal).
CHILDSUSP pid sigName msg
This format is used when a child process has been sus-
pended because of a signal. The second element of error-
Code will be the process's identifier, in decimal. The
third element will be the symbolic name of the signal
that caused the process to suspend; this will be one of
the names from the include file signal.h, such as SIGT-
TIN. The fourth element will be a short human-readable
message describing the signal, such as ``background tty
read'' for SIGTTIN.
NONE This format is used for errors where no additional infor-
mation is available for an error besides the message
returned with the error. In these cases errorCode will
consist of a list containing a single element whose con-
tents are NONE.
POSIX errName msg
If the first element of errorCode is POSIX, then the
error occurred during a POSIX kernel call. The second
element of the list will contain the symbolic name of the
error that occurred, such as ENOENT; this will be one of
the values defined in the include file errno.h. The
third element of the list will be a human-readable mes-
sage corresponding to errName, such as ``no such file or
directory'' for the ENOENT case.
To set errorCode, applications should use library procedures
such as Tcl_SetErrorCode and Tcl_PosixError, or they may invoke
the error command. If one of these methods hasn't been used,
then the Tcl interpreter will reset the variable to NONE after
the next error.
errorInfo
After an error has occurred, this string will contain one or
more lines identifying the Tcl commands and procedures that were
being executed when the most recent error occurred. Its con-
tents take the form of a stack trace showing the various nested
Tcl commands that had been invoked at the time of the error.
tcl_library
This variable holds the name of a directory containing the sys-
tem library of Tcl scripts, such as those used for auto-loading.
The value of this variable is returned by the info library com-
mand. See the library manual entry for details of the facili-
ties provided by the Tcl script library. Normally each applica-
tion or package will have its own application-specific script
library in addition to the Tcl script library; each application
should set a global variable with a name like $app_library
(where app is the application's name) to hold the network file
name for that application's library directory. The initial
value of tcl_library is set when an interpreter is created by
searching several different directories until one is found that
contains an appropriate Tcl startup script. If the TCL_LIBRARY
environment variable exists, then the directory it names is
checked first. If TCL_LIBRARY isn't set or doesn't refer to an
appropriate directory, then Tcl checks several other directories
based on a compiled-in default location, the location of the
binary containing the application, and the current working
directory.
tcl_patchLevel
When an interpreter is created Tcl initializes this variable to
hold a string giving the current patch level for Tcl, such as
7.3p2 for Tcl 7.3 with the first two official patches, or 7.4b4
for the fourth beta release of Tcl 7.4. The value of this vari-
able is returned by the info patchlevel command.
tcl_pkgPath
This variable holds a list of directories indicating where pack-
ages are normally installed. It is not used on Windows. It
typically contains either one or two entries; if it contains two
entries, the first is normally a directory for platform-depen-
dent packages (e.g., shared library binaries) and the second is
normally a directory for platform-independent packages (e.g.,
script files). Typically a package is installed as a subdirec-
tory of one of the entries in $tcl_pkgPath. The directories in
$tcl_pkgPath are included by default in the auto_path variable,
so they and their immediate subdirectories are automatically
searched for packages during package require commands. Note:
tcl_pkgPath it not intended to be modified by the application.
Its value is added to auto_path at startup; changes to tcl_pkg-
Path are not reflected in auto_path. If you want Tcl to search
additional directories for packages you should add the names of
those directories to auto_path, not tcl_pkgPath.
tcl_platform
This is an associative array whose elements contain information
about the platform on which the application is running, such as
the name of the operating system, its current release number,
and the machine's instruction set. The elements listed below
will always be defined, but they may have empty strings as val-
ues if Tcl couldn't retrieve any relevant information. In addi-
tion, extensions and applications may add additional values to
the array. The predefined elements are:
byteOrder
The native byte order of this machine: either littleEn-
dian or bigEndian.
debug If this variable exists, then the interpreter was com-
piled with and linked to a debug-enabled C run-time.
This variable will only exist on Windows, so extension
writers can specify which package to load depending on
the C run-time library that is in use. This is not an
indication that this core contains symbols.
machine
The instruction set executed by this machine, such as
intel, PPC, 68k, or sun4m. On UNIX machines, this is the
value returned by uname -m.
os The name of the operating system running on this machine,
such as Windows 95, Windows NT, MacOS, or SunOS. On UNIX
machines, this is the value returned by uname -s. On
Windows 95 and Windows 98, the value returned will be
Windows 95 to provide better backwards compatibility to
Windows 95; to distinguish between the two, check the
osVersion.
osVersion
The version number for the operating system running on
this machine. On UNIX machines, this is the value
returned by uname -r. On Windows 95, the version will be
4.0; on Windows 98, the version will be 4.10.
platform
Either windows, macintosh, or unix. This identifies the
general operating environment of the machine.
threaded
If this variable exists, then the interpreter was com-
piled with threads enabled.
user This identifies the current user based on the login
information available on the platform. This comes from
the USER or LOGNAME environment variable on Unix, and the
value from GetUserName on Windows and Macintosh.
wordSize
This gives the size of the native-machine word in bytes
(strictly, it is same as the result of evaluating
sizeof(long) in C.)
tcl_precision
This variable controls the number of digits to generate when
converting floating-point values to strings. It defaults to 12.
17 digits is ``perfect'' for IEEE floating-point in that it
allows double-precision values to be converted to strings and
back to binary with no loss of information. However, using 17
digits prevents any rounding, which produces longer, less intu-
itive results. For example, expr 1.4 returns 1.3999999999999999
with tcl_precision set to 17, vs. 1.4 if tcl_precision is 12.
All interpreters in a process share a single tcl_precision
value: changing it in one interpreter will affect all other
interpreters as well. However, safe interpreters are not
allowed to modify the variable.
tcl_rcFileName
This variable is used during initialization to indicate the name
of a user-specific startup file. If it is set by application-
specific initialization, then the Tcl startup code will check
for the existence of this file and source it if it exists. For
example, for wish the variable is set to ~/.wishrc for Unix and
~/wishrc.tcl for Windows.
tcl_rcRsrcName
This variable is only used on Macintosh systems. The variable
is used during initialization to indicate the name of a user-
specific TEXT resource located in the application or extension
resource forks. If it is set by application-specific initial-
ization, then the Tcl startup code will check for the existence
of this resource and source it if it exists. For example, the
Macintosh wish application has the variable is set to tclshrc.
tcl_traceCompile
The value of this variable can be set to control how much trac-
ing information is displayed during bytecode compilation. By
default, tcl_traceCompile is zero and no information is dis-
played. Setting tcl_traceCompile to 1 generates a one-line sum-
mary in stdout whenever a procedure or top-level command is com-
piled. Setting it to 2 generates a detailed listing in stdout
of the bytecode instructions emitted during every compilation.
This variable is useful in tracking down suspected problems with
the Tcl compiler. It is also occasionally useful when convert-
ing existing code to use Tcl8.0.
This variable and functionality only exist if TCL_COMPILE_DEBUG was
defined during Tcl's compilation.
tcl_traceExec
The value of this variable can be set to control how much trac-
ing information is displayed during bytecode execution. By
default, tcl_traceExec is zero and no information is displayed.
Setting tcl_traceExec to 1 generates a one-line trace in stdout
on each call to a Tcl procedure. Setting it to 2 generates a
line of output whenever any Tcl command is invoked that contains
the name of the command and its arguments. Setting it to 3 pro-
duces a detailed trace showing the result of executing each
bytecode instruction. Note that when tcl_traceExec is 2 or 3,
commands such as set and incr that have been entirely replaced
by a sequence of bytecode instructions are not shown. Setting
this variable is useful in tracking down suspected problems with
the bytecode compiler and interpreter. It is also occasionally
useful when converting code to use Tcl8.0.
This variable and functionality only exist if TCL_COMPILE_DEBUG was
defined during Tcl's compilation.
tcl_wordchars
The value of this variable is a regular expression that can be
set to control what are considered ``word'' characters, for
instances like selecting a word by double-clicking in text in
Tk. It is platform dependent. On Windows, it defaults to \S,
meaning anything but a Unicode space character. Otherwise it
defaults to \w, which is any Unicode word character (number,
letter, or underscore).
tcl_nonwordchars
The value of this variable is a regular expression that can be
set to control what are considered ``non-word'' characters, for
instances like selecting a word by double-clicking in text in
Tk. It is platform dependent. On Windows, it defaults to \s,
meaning any Unicode space character. Otherwise it defaults to
\W, which is anything but a Unicode word character (number, let-
ter, or underscore).
tcl_version
When an interpreter is created Tcl initializes this variable to
hold the version number for this version of Tcl in the form x.y.
Changes to x represent major changes with probable incompatibil-
ities and changes to y represent small enhancements and bug
fixes that retain backward compatibility. The value of this
variable is returned by the info tclversion command.
OTHER GLOBAL VARIABLES
The following variables are only guaranteed to exist in tclsh and wish
executables; the Tcl library does not define them itself but many Tcl
environments do.
argc The number of arguments to tclsh or wish.
argv Tcl list of arguments to tclsh or wish.
argv0 The script that tclsh or wish started executing (if it was speci-
fied) or otherwise the name by which tclsh or wish was invoked.
tcl_interactive
Contains 1 if tclsh or wish is running interactively (no script
was specified and standard input is a terminal-like device), 0
otherwise.
The wish executably additionally specifies the following global vari-
able:
geometry
If set, contains the user-supplied geometry specification to use
for the main Tk window.
SEE ALSO
eval(n), tclsh(1), wish(1)
KEYWORDS
arithmetic, bytecode, compiler, error, environment, POSIX, precision,
subprocess, variables
Tcl 8.0 tclvars(n)
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