path: root/contrib/tcl/doc/tclvars.n

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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" 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