tclvars(3tcl) Tcl Built-In Commands tclvars(3tcl)
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NAME
argc, argv, argv0, auto_path, env, errorCode, errorInfo, tcl_interac-
tive, tcl_library, tcl_nonwordchars, tcl_patchLevel, tcl_pkgPath,
tcl_platform, tcl_precision, tcl_rcFileName, tcl_traceCompile,
tcl_traceExec, tcl_wordchars, tcl_version - 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 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 in-
dex 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 global variable, the parent
directory of tcl_library, the directories listed in the tcl_pkg-
Path variable. Additional locations to look for files and pack-
age indices should normally be added to this variable using lap-
pend.
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 di-
rectories to search during auto-load operations. Direc-
tories must be specified in Tcl format, using “/” as the
path separator, regardless of platform. This variable is
only used when initializing the auto_path variable.
env(TCL_TZ), env(TZ)
These specify the default timezone used for parsing and
formatting times and dates in the clock command. On many
platforms, the TZ environment variable is set up by the
operating system.
env(LC_ALL), env(LC_MESSAGES), env(LANG)
These environment variables are used by the msgcat pack-
age to determine what locale to format messages using.
env(TCL_INTERP_DEBUG_FRAME)
If existing, it has the same effect as running interp de-
bug {} -frame 1 as the very first command of each new Tcl
interpreter.
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 er-
ror 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 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 hu-
man-readable description of the error. Code will be ei-
ther 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 re-
turned with the error. In these cases the -errorcode re-
turn 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 hu-
man-readable message corresponding to errName, such as
“no such file or directory” for the ENOENT case.
TCL ...
Indicates some sort of problem generated in relation to
Tcl itself, e.g. a failure to look up a channel or vari-
able.
To set the -errorcode return option, applications should use li-
brary procedures such as Tcl_SetObjErrorCode, Tcl_SetReturnOp-
tions, and Tcl_PosixError, or they may invoke the -errorcode op-
tion of the return command. If none of these methods for set-
ting the error code has been used, the Tcl interpreter will re-
set 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 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 bi-
nary containing the application, and the current working direc-
tory.
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 the tcl_pkgPath variable. The di-
rectories in the tcl_pkgPath variable are included by default in
the auto_path variable, so they and their immediate subdirecto-
ries are automatically searched for packages during package re-
quire commands. Note: tcl_pkgPath is not intended to be modi-
fied 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 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 ad-
dition, 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.
engine The name of the Tcl language implementation. When the
interpreter is first created, this is always set to the
string Tcl.
machine
The instruction set executed by this machine, such as in-
tel, 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 NT or SunOS. On UNIX machines, this is
the value returned by uname -s.
osVersion
The version number for the operating system running on
this machine. On UNIX machines, this is the value re-
turned by uname -r.
pathSeparator
The character that should be used to split PATH-like en- │
vironment variables into their corresponding list of di- │
rectory names.
platform
Either windows, or unix. This identifies the general op-
erating 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 in-
formation available on the platform. This value comes
from the getuid() and getpwuid() system calls on Unix,
and the value from the GetUserName() system call on Win-
dows.
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.
If tcl_precision is not zero, then when Tcl converts a floating
point number, it creates a decimal representation of at most
tcl_precision significant digits; the result may be shorter if
the shorter result represents the original number exactly. If no
result of at most tcl_precision digits is an exact representa-
tion of the original number, the one that is closest to the
original number is chosen. If the original number lies pre-
cisely between two equally accurate decimal representations,
then the one with an even value for the least significant digit
is chosen; for instance, if tcl_precision is 3, then 0.3125 will
convert to 0.312, not 0.313, while 0.6875 will convert to 0.688,
not 0.687. Any string of trailing zeroes that remains is
trimmed.
a tcl_precision value of 17 digits is “perfect” for IEEE float-
ing-point in that it allows double-precision values to be con-
verted to strings and back to binary with no loss of informa-
tion. For this reason, you will often see it as a value in
legacy code that must run on Tcl versions before 8.5. It is no
longer recommended; as noted above, a zero value is the pre-
ferred method.
All interpreters in a thread share a single tcl_precision value:
changing it in one interpreter will affect all other inter-
preters as well. Safe interpreters are not allowed to modify
the variable.
Valid values for tcl_precision range from 0 to 17.
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 de-
fault, 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 in-
stances like selecting a word by double-clicking in text in Tk.
It is platform dependent. On Windows, it defaults to \S, mean-
ing anything but a Unicode space character. Otherwise it de-
faults to \w, which is any Unicode word character (number, let-
ter, 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.
EXAMPLES
To add a directory to the collection of locations searched by package
require, e.g., because of some application-specific packages that are
used, the auto_path variable needs to be updated:
lappend ::auto_path [file join [pwd] "theLibDir"]
A simple though not very robust way to handle command line arguments of
the form “-foo 1 -bar 2” is to load them into an array having first
loaded in the default settings:
array set arguments {-foo 0 -bar 0 -grill 0}
array set arguments $::argv
puts "foo is $arguments(-foo)"
puts "bar is $arguments(-bar)"
puts "grill is $arguments(-grill)"
The argv0 global variable can be used (in conjunction with the info
script command) to determine whether the current script is being exe-
cuted as the main script or loaded as a library. This is useful be-
cause it allows a single script to be used as both a library and a
demonstration of that library:
if {$::argv0 eq [info script]} {
# running as: tclsh example.tcl
} else {
package provide Example 1.0
}
SEE ALSO
eval(3tcl), library(3tcl), tclsh(1), tkvars(3tcl), wish(1)
KEYWORDS
arithmetic, bytecode, compiler, error, environment, POSIX, precision,
subprocess, user, variables
Tcl 8.0 tclvars(3tcl)
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