Provided by: tcl8.5-doc_8.5.19-4_all 
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 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 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
platforms 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 therefore 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 variable is
only used when initializing the auto_path variable.
env(TCL_INTERP_DEBUG_FRAME)
If existing, it has the same effect as running interp debug {} -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 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 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 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 -errorcode 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 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 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 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 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 number, 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 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 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.
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 platform. This value comes from the getuid() and getpwuid() system
calls on Unix, and the value from the GetUserName() system call 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.
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 representation of the
original number, the one that is closest to the original number is chosen. If the
original number lies precisely 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 floating-point in that it
allows double-precision values to be converted to strings and back to binary with
no loss of information. 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 preferred method.
All interpreters in a thread share a single tcl_precision value: changing it in one
interpreter will affect all other interpreters 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 tracing information is
displayed during bytecode compilation. By default, tcl_traceCompile is zero and no
information is displayed. Setting tcl_traceCompile to 1 generates a one-line
summary in stdout whenever a procedure or top-level command is compiled. 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 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 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 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 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, 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 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 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(3tcl), tclsh(1), wish(1)
KEYWORDS
arithmetic, bytecode, compiler, error, environment, POSIX, precision, subprocess,
variables