tclvars(n) Tcl Built-In Commands tclvars(n)______________________________________________________________________________NAMEtclvars - 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.
auto_path
If set, then it must contain a valid Tcl list giving directories
to search during auto-load operations (including for package
index files when using the default package unknown handler).
This variable is initialized during startup to contain, in
order: the directories listed in the TCLLIBPATH environment
variable, the directory named by the $tcl_library variable, the
parent directory of $tcl_library, the directories listed in the
$tcl_pkgPath variable. Additional locations to look for files
and package indices should normally be added to this variable
using lappend.
Additional variables relating to package management exist. More
details are listed in the VARIABLES section of the library man‐
ual page.
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 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 moni‐
toring 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 operating
system as “path”, “Path”, “PaTh”, etc., causing otherwise simple
Tcl code to have to support many special cases. All other envi‐
ronment variables inherited by Tcl are left unmodified. 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.
The following elements of env are special to Tcl:
env(HOME)
This environment variable, if set, gives the location of
the directory considered to be the current user's home
directory, and to which a call of cd without arguments or
with just “~” as an argument will change into. Most plat‐
forms set this correctly by default; it does not normally
need to be set by user code.
env(TCL_LIBRARY)
If set, then it specifies the location of the directory
containing library scripts (the value of this variable
will be assigned to the tcl_library variable and there‐
fore returned by the command info library). If this
variable is not set then a default value is used.
Note that this environment variable should not normally
be set.
env(TCLLIBPATH)
If set, then it must contain a valid Tcl list giving
directories to search during auto-load operations.
Directories must be specified in Tcl format, using “/” as
the path separator, regardless of platform. This vari‐
able is only used when initializing the auto_path vari‐
able.
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 follow‐
ing formats for -errorcode return options are used by the Tcl
core; individual applications 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 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 underly‐
ing 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 ele‐
ment 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 sus‐
pended 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 ele‐
ment 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 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_SetReturnOp‐
tions, and Tcl_PosixError, or they may invoke the -errorcode
option of the return command. If none of these methods for set‐
ting the error code has been used, the Tcl interpreter will
reset the variable to NONE after the next error.
TCL ...
Indicates some sort of problem generated in relation to Tcl
itself, e.g. a failure to look up a channel or variable.
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 identifying 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 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 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 current 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 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 is 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 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 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, 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.
pathSeparator
The character that should be used to split PATH-like │
environment variables into their corresponding list of │
directory names.
platform
Either windows, or unix. This identifies the general
operating environment of the machine.
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.)
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.
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 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 con‐
vert numbers using as few digits as possible while still distin‐
guishing any floating point number from its nearest neighbours.
It differs from using an arbitrarily high value for tcl_preci‐
sion 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 interpreter will affect all other inter‐
preters 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 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.
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.
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 executable additionally specifies the following global vari‐
able:
geometry
If set, contains the user-supplied geometry specification to use
for the main Tk window.
SEE ALSOeval(n), library(n), tclsh(1), wish(1)KEYWORDS
arithmetic, bytecode, compiler, error, environment, POSIX, precision,
subprocess, variables
Tcl 8.0 tclvars(n)
