source: downloads/tcl8.5.2/doc/catch.n @ 35

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'\" Copyright (c) 1993-1994 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\" Contributions from Don Porter, NIST, 2003.  (not subject to US copyright)
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\" RCS: @(#) $Id: catch.n,v 1.18 2007/12/13 15:22:32 dgp Exp $
'\" 
.so man.macros
.TH catch n "8.5" Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
catch \- Evaluate script and trap exceptional returns
.SH SYNOPSIS
\fBcatch\fI script \fR?\fIresultVarName\fR? ?\fIoptionsVarName\fR?
.BE

.SH DESCRIPTION
.PP
The \fBcatch\fR command may be used to prevent errors from aborting command
interpretation.  The \fBcatch\fR command calls the Tcl interpreter recursively to
execute \fIscript\fR, and always returns without raising an error,
regardless of any errors that might occur while executing \fIscript\fR.
.PP
If \fIscript\fR raises an error, \fBcatch\fR will return a non-zero integer
value corresponding to the exceptional return code returned by evaluation
of \fIscript\fR.  Tcl defines the normal return code from script
evaluation to be zero (0), or \fBTCL_OK\fR.  Tcl also defines four exceptional
return codes: 1 (\fBTCL_ERROR\fR), 2 (\fBTCL_RETURN\fR), 3 (\fBTCL_BREAK\fR),
and 4 (\fBTCL_CONTINUE\fR).  Errors during evaluation of a script are indicated
by a return code of \fBTCL_ERROR\fR.  The other exceptional return codes are
returned by the \fBreturn\fR, \fBbreak\fR, and \fBcontinue\fR commands
and in other special situations as documented.  Tcl packages can define
new commands that return other integer values as return codes as well,
and scripts that make use of the \fBreturn -code\fR command can also
have return codes other than the five defined by Tcl.
.PP
If the \fIresultVarName\fR argument is given, then the variable it names is
set to the result of the script evaluation.  When the return code from
the script is 1 (\fBTCL_ERROR\fR), the value stored in \fIresultVarName\fR is an error
message.  When the return code from the script is 0 (\fBTCL_OK\fR), the value
stored in \fIresultVarName\fR is the value returned from \fIscript\fR.
.PP
.VS 8.5
If the \fIoptionsVarName\fR argument is given, then the variable it
names is set to a dictionary of return options returned by evaluation
of \fIscript\fR.  Tcl specifies two entries that are always 
defined in the dictionary: \fB\-code\fR and \fB\-level\fR.  When
the return code from evaluation of \fIscript\fR is not \fBTCL_RETURN\fR,
the value of the \fB\-level\fR entry will be 0, and the value
of the \fB\-code\fR entry will be the same as the return code.
Only when the return code is \fBTCL_RETURN\fR will the values of
the \fB\-level\fR and \fB\-code\fR entries be something else, as
further described in the documentation for the \fBreturn\fR command.
.PP
When the return code from evaluation of \fIscript\fR is \fBTCL_ERROR\fR,
three additional entries are defined in the dictionary of return options
stored in \fIoptionsVarName\fR: \fB\-errorinfo\fR, \fB\-errorcode\fR, 
and \fB\-errorline\fR.  The value of the \fB\-errorinfo\fR entry
is a formatted stack trace containing more information about
the context in which the error happened.  The formatted stack
trace is meant to be read by a person.  The value of
the \fB\-errorcode\fR entry is additional information about the
error stored as a list.  The \fB\-errorcode\fR value is meant to
be further processed by programs, and may not be particularly
readable by people.  The value of the \fB\-errorline\fR entry
is an integer indicating which line of \fIscript\fR was being
evaluated when the error occurred.  The values of the \fB\-errorinfo\fR
and \fB\-errorcode\fR entries of the most recent error are also
available as values of the global variables \fB::errorInfo\fR
and \fB::errorCode\fR respectively.
.PP
Tcl packages may provide commands that set other entries in the
dictionary of return options, and the \fBreturn\fR command may be
used by scripts to set return options in addition to those defined
above.
.VE 8.5
.SH EXAMPLES
The \fBcatch\fR command may be used in an \fBif\fR to branch based on
the success of a script.
.CS
if { [\fBcatch\fR {open $someFile w} fid] } {
    puts stderr "Could not open $someFile for writing\en$fid"
    exit 1
}
.CE
.PP
There are more complex examples of \fBcatch\fR usage in the
documentation for the \fBreturn\fR command.

.SH "SEE ALSO" 
break(n), continue(n), dict(n), error(n), return(n), tclvars(n)

.SH KEYWORDS
catch, error