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+'\"
+'\" Copyright (c) 1993 The Regents of the University of California.
+'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
+'\"
+'\" See the file "license.terms" for information on usage and redistribution
+'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+'\" 
+'\" SCCS: @(#) tclvars.n 1.15 96/04/12 08:28:20
+'\" 
+.so man.macros
+.TH tclvars n 7.5 Tcl "Tcl Built-In Commands"
+.BS
+'\" Note:  do not modify the .SH NAME line immediately below!
+.SH NAME
+tclvars \- Variables used by Tcl
+.BE
+
+.SH DESCRIPTION
+.PP
+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.
+.TP
+\fBenv\fR
+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 \fBenv\fR will remove the corresponding
+environment variable.
+Changes to the \fBenv\fR array will affect the environment
+passed to children by commands like \fBexec\fR.
+If the entire \fBenv\fR array is unset then Tcl will stop
+monitoring \fBenv\fR accesses and will not update environment
+variables.
+.TP
+\fBerrorCode\fR
+After an error has occurred, this variable will be set to hold
+additional information about the error in a form that is easy
+to process with programs.
+\fBerrorCode\fR consists of a Tcl list with one or more elements.
+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 \fBerrorCode\fR are used by the
+Tcl core; individual applications may define additional formats.
+.RS
+.TP
+\fBARITH\fI code msg\fR
+.VS
+This format is used when an arithmetic error occurs (e.g. an attempt
+to divide by zero in the \fBexpr\fR command).
+\fICode\fR identifies the precise error and \fImsg\fR provides a
+human-readable description of the error.  \fICode\fR 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).
+.VE
+.TP
+\fBCHILDKILLED\fI pid sigName msg\fR
+This format is used when a child process has been killed because of
+a signal.  The second element of \fBerrorCode\fR 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 \fBSIGPIPE\fR.
+The fourth element will be a short human-readable message
+describing the signal, such as ``write on pipe with no readers''
+for \fBSIGPIPE\fR.
+.TP
+\fBCHILDSTATUS\fI pid code\fR
+This format is used when a child process has exited with a non-zero
+exit status.  The second element of \fBerrorCode\fR will be the
+process's identifier (in decimal) and the third element will be the exit
+code returned by the process (also in decimal).
+.TP
+\fBCHILDSUSP\fI pid sigName msg\fR
+This format is used when a child process has been suspended because
+of a signal.
+The second element of \fBerrorCode\fR 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 \fBSIGTTIN\fR.
+The fourth element will be a short human-readable message
+describing the signal, such as ``background tty read''
+for \fBSIGTTIN\fR.
+.TP
+\fBNONE\fR
+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 \fBerrorCode\fR will consist of a list
+containing a single element whose contents are \fBNONE\fR.
+.TP
+\fBPOSIX \fIerrName msg\fR
+.VS
+If the first element of \fBerrorCode\fR is \fBPOSIX\fR, then
+the error occurred during a POSIX kernel call.
+.VE
+The second element of the list will contain the symbolic name
+of the error that occurred, such as \fBENOENT\fR; 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 \fIerrName\fR, such as
+``no such file or directory'' for the \fBENOENT\fR case.
+.PP
+To set \fBerrorCode\fR, applications should use library
+procedures such as \fBTcl_SetErrorCode\fR and
+.VS
+\fBTcl_PosixError\fR,
+.VE
+or they may invoke the \fBerror\fR command.
+If one of these methods hasn't been used, then the Tcl
+interpreter will reset the variable to \fBNONE\fR after
+the next error.
+.RE
+.TP
+\fBerrorInfo\fR
+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.
+.TP
+\fBtcl_library\fR
+.VS
+This variable holds the network 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 \fBinfo library\fR command.
+See the \fBlibrary\fR manual entry for details of the facilities 
+rovided 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
+\fB$\fIapp\fB_library\fR (where \fIapp\fR is the application's name)
+to hold the network file name for that application's library directory.
+The initial value of \fBtcl_library\fR 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 \fBTCL_LIBRARY\fR environment variable exists, then
+the directory it names is checked first.
+If \fBTCL_LIBRARY\fR 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.
+.VE
+.TP
+\fBtcl_patchLevel\fR
+When an interpreter is created Tcl initializes this variable to
+hold a string giving the current patch level for Tcl, such as
+\fB7.3p2\fR for Tcl 7.3 with the first two official patches, or
+\fB7.4b4\fR for the fourth beta release of Tcl 7.4.
+The value of this variable is returned by the \fBinfo patchlevel\fR
+command.
+.VS br
+.TP
+\fBtcl_platform\fR
+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:
+.RS
+.TP
+\fBmachine\fR
+The instruction set executed by this machine, such as
+\fBPPC\fR, \fB68k\fR, or \fBsun4m\fR.  On UNIX machines, this
+is the value returned by \fBuname -m\fR.
+.TP
+\fBos\fR
+The name of the operating system running on this machine, such
+as \fBWin95\fR, \fBMacOS\fR, or \fBSunOS\fR.  On UNIX machines,
+this is the value returned by \fBuname -s\fR.
+.TP
+\fBosVersion\fR
+The version number for the operating system running on this machine.
+On UNIX machines, this is the value returned by \fBuname -r\fR.
+.TP
+\fBplatform\fR
+Either \fBwindows\fR, \fBmacintosh\fR, or \fBunix\fR.  This identifies the
+general operating environment of the machine.
+.RE
+.VE
+.TP
+\fBtcl_precision\fR
+If this variable is set, it must contain a decimal number giving the
+number of significant digits to include when converting floating-point
+values to strings.
+If this variable is not set then 6 digits are included.
+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 precision.
+.VS br
+.TP
+\fBtcl_rcFileName\fR
+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 \fBsource\fR it if it exists.  For example, for \fBwish\fR
+the variable is set to \fB~/.wishrc\fR.
+.VE
+.TP
+\fBtcl_version\fR
+When an interpreter is created Tcl initializes this variable to
+hold the version number for this version of Tcl in the form \fIx.y\fR.
+Changes to \fIx\fR represent major changes with probable
+incompatibilities and changes to \fIy\fR represent small enhancements and
+bug fixes that retain backward compatibility.
+The value of this variable is returned by the \fBinfo tclversion\fR
+command.
+
+.SH KEYWORDS
+arithmetic, error, environment, POSIX, precision, subprocess, variables