tclvars.n

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'\" RCS: @(#) $Id: tclvars.n,v 1.13 2002/07/01 18:24:39 jenglish Exp $
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.so man.macros
.TH tclvars n 8.0 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.
.RS
.VS 8.0 
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.
.VE
.RE
.RS
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:
.TP
\fBLOGIN\fR
This holds the Chooser name of the Macintosh.
.TP
\fBUSER\fR
This also holds the Chooser name of the Macintosh.
.TP
\fBSYS_FOLDER\fR
The path to the system directory.
.TP
\fBAPPLE_M_FOLDER\fR
The path to the Apple Menu directory.
.TP
\fBCP_FOLDER\fR
The path to the control panels directory.
.TP
\fBDESK_FOLDER\fR
The path to the desk top directory.
.TP
\fBEXT_FOLDER\fR
The path to the system extensions directory.
.TP
\fBPREF_FOLDER\fR
The path to the preferences directory.
.TP
\fBPRINT_MON_FOLDER\fR
The path to the print monitor directory.
.TP
\fBSHARED_TRASH_FOLDER\fR
The path to the network trash directory.
.TP
\fBTRASH_FOLDER\fR
The path to the trash directory.
.TP
\fBSTART_UP_FOLDER\fR
The path to the start up directory.
.TP
\fBHOME\fR
The path to the application's default directory.
.PP
You can also create your own environment variables for the Macintosh.
A file named  \fITcl Environment Variables\fR may be placed in the
preferences folder in the Mac system folder.  Each line of this file
should be of the form \fIVAR_NAME=var_data\fR.
.PP
The last alternative is to place environment variables in a 'STR#' 
resource named \fITcl Environment Variables\fR 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 \fItclMacEnv.c\fR 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.
.RE
.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
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).
.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
If the first element of \fBerrorCode\fR is \fBPOSIX\fR, 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 \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 \fBTcl_PosixError\fR,
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
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 \fBinfo library\fR command.
See the \fBlibrary\fR 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
\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