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  4. tclvars(n)

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tclvars(n) Tcl Built-In Commands tclvars(n) 





NAME


tclvars - 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 normally be treated as read-only by application-specific code and by
    users.



  

  
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.





  

  
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.







  

  
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.





  

  
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.

  

  
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).

  

  
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.

  

  
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.

  

  
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.





  

  
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.

  

  
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.

  

  
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.

  

  
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.

  

  
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:







  

  
The native byte order of this machine: either littleEndian or
      bigEndian.

  

  
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.

  

  
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.

  

  
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.

  

  
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.

  

  
Either windows, or unix. This identifies the general
      operating environment of the machine.

  

  
If this variable exists, then the interpreter was compiled with threads
      enabled.

  

  
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.

  

  
This gives the size of the native-machine word in bytes (strictly, it is
      same as the result of evaluating sizeof(long) in C.)

  

  
This gives the size of the native-machine pointer in bytes (strictly, it
      is same as the result of evaluating sizeof(void*) in C.)







  

  
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 interpreter will affect all
  other interpreters as well. However, safe interpreters are not allowed to
  modify the variable.



  

  
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.

  

  
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.



  

  
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.



  

  
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).

  

  
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).

  

  
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.



  

  
The number of arguments to tclsh or wish.

  

  
Tcl list of arguments to tclsh or wish.

  

  
The script that tclsh or wish started executing (if it was
      specified) or otherwise the name by which tclsh or wish was
      invoked.

  

  
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:



  

  
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







  

    8.0
    Tcl