tclvars(n) manual page
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tclvars - Variables used by Tcl
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
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
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 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 variables 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 Macintosh. 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 application. 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 a list value representing 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
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 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 suspended 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 suspend; this will be one
of the names from the include file signal.h, such as SIGTTIN. 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 information 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 contents 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 message 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 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 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 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 it 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 couldn’t 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.
- 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
compiled 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 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 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 initialization,
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 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. It is also occasionally useful
when converting 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 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. 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, 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.
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 executably additionally specifies the following global variable:
- geometry
- If set, contains the user-supplied geometry specification to use
for the main Tk window.
eval(n)
, tclsh(1)
, wish(1)
arithmetic,
bytecode, compiler, error, environment, POSIX, precision, subprocess, variables
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