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tclvars(n) Tcl Built-In Commands tclvars(n)
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NAME
tclvars - Variables used by Tcl
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DESCRIPTION
The following global variables are created and managed automatically by the Tcl library. Except
where noted below, these variables should normally 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 element will return the value of the corresponding environment
variable. Setting an element of the array will modify the corresponding environment variable
or create a new one if it does not 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 variables.
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 operat-ing operating
ing system as "path", "Path", "PaTh", etc., causing otherwise simple Tcl code to have to sup-port support
port many special cases. All other environment variables inherited by Tcl are left unmodi-fied. unmodified.
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 will not work on Windows and is discouraged for cross-platform
usage.
errorCode
This variable holds the value of the -errorcode return option set by the most recent error
that occurred in this interpreter. This list value represents additional information about
the error in a form that is easy to process with programs. 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 return options are used by the Tcl core; individual applica-tions applications
tions may define additional formats.
ARITH code msg
This format is used when an arithmetic error occurs (e.g. an attempt to divide zero by
zero in the expr command). Code identifies the precise error and msg provides a human-readable humanreadable
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).
Detection of these errors depends in part on the underlying hardware and system
libraries.
CHILDKILLED pid sigName msg
This format is used when a child process has been killed because of a signal. The pid
element will be the process's identifier (in decimal). The sigName 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 msg 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
pid element will be the process's identifier (in decimal) and the code 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 suspended because of a signal. The
pid element will be the process's identifier, in decimal. The sigName 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 SIGTTIN. The msg 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 information is available for an
error besides the message returned with the error. In these cases the -errorcode
return option will consist of a list containing a single element whose contents are
NONE.
POSIX errName msg
If the first element is POSIX, then the error occurred during a POSIX kernel call. The
errName element 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 msg
element will be a human-readable message corresponding to errName, such as "no such
file or directory" for the ENOENT case.
To set the -errorcode return option, applications should use library procedures such as
Tcl_SetObjErrorCode, Tcl_SetReturnOptions, and Tcl_PosixError, or they may invoke the -error-code -errorcode
code option of the return command. If none of these methods for setting the error code has
been used, the Tcl interpreter will reset the variable to NONE after the next error.
errorInfo
This variable holds the value of the -errorinfo return option set by the most recent error
that occurred in this interpreter. This string value will contain one or more lines identify-ing identifying
ing the Tcl commands and procedures that were being executed when the most recent error
occurred. Its contents 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 system library of Tcl scripts, such
as those used for auto-loading. The value of this variable is returned by the info library
command. See the library manual entry for details of the facilities provided by the Tcl
script library. Normally each application 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 net-work network
work 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 vari-able variable
able exists, then the directory it names is checked first. If TCL_LIBRARY is not 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 cur-rent current
rent patch level for Tcl, such as 8.4.16 for Tcl 8.4 with the first sixteen official patches,
or 8.5b3 for the third beta release of Tcl 8.5. The value of this variable is returned by the
info patchlevel command.
tcl_pkgPath
This variable holds a list of directories indicating where packages 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-dependent 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 subdirectory 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 is not intended to be modified by the
application. Its value is added to auto_path at startup; changes to tcl_pkgPath 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 num-ber, number,
ber, and the machine's instruction set. The elements listed below will always be defined, but
they may have empty strings as values if Tcl could not retrieve any relevant information. In
addition, extensions and applications may add additional values to the array. The predefined
elements are:
byteOrder
The native byte order of this machine: either littleEndian or bigEndian.
debug If this variable exists, then the interpreter was compiled with and linked to a debug-enabled debugenabled
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, 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 com-patibility compatibility
patibility 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, or unix. This identifies the general operating environment of the
machine.
threaded
If this variable exists, then the interpreter was compiled with threads enabled.
user This identifies the current user based on the login information available on the plat-form. platform.
form. This comes from the USER or LOGNAME environment variable on Unix, and the value
from GetUserName on Windows.
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.)
pointerSize
This gives the size of the native-machine pointer in bytes (strictly, it is same as the
result of evaluating sizeof(void*) in C.)
tcl_precision
This variable controls the number of digits to generate when converting floating-point values
to strings. It defaults to 0. Applications should not change this value; it is provided for |
compatibility with legacy code. |
The default value of 0 is special, meaning that Tcl should convert numbers using as few digits |
as possible while still distinguishing any floating point number from its nearest neighbours. |
It differs from using an arbitrarily high value for tcl_precision in that an inexact number |
like 1.4 will convert as 1.4 rather than 1.3999999999999999 even though the latter is nearer |
to the exact value of the binary number. |
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 intuitive 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 thread share a single tcl_precision value: changing it in one inter-preter interpreter
preter 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_traceCompile
The value of this variable can be set to control how much tracing information is displayed
during bytecode compilation. By default, tcl_traceCompile is zero and no information is dis-played. displayed.
played. Setting tcl_traceCompile to 1 generates a one-line summary in stdout whenever a pro-cedure procedure
cedure or top-level command is compiled. Setting it to 2 generates a detailed listing in std-out stdout
out of the bytecode instructions emitted during every compilation. This variable is useful in
tracking down suspected problems with the Tcl compiler.
This variable and functionality only exist if TCL_COMPILE_DEBUG was defined during Tcl's com-pilation. compilation.
pilation.
tcl_traceExec
The value of this variable can be set to control how much tracing 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 proce-dure. procedure.
dure. 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 produces 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 prob-lems problems
lems with the bytecode compiler and interpreter.
This variable and functionality only exist if TCL_COMPILE_DEBUG was defined during Tcl's com-pilation. compilation.
pilation.
tcl_wordchars
The value of this variable is a regular expression that can be set to control what are consid-ered considered
ered "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 consid-ered considered
ered "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 char-acter. character.
acter. Otherwise it defaults to \W, which is anything but a Unicode word character (number,
letter, 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
incompatibilities 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 com-mand. command.
mand.
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 specified) 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 executable additionally specifies the following global variable:
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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