'\" '\" Copyright (c) 1993-1994 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. '\" '\" RCS: @(#) $Id: catch.n,v 1.1.1.1 2007/07/10 15:04:23 duncan Exp $ '\" .so man.macros .TH catch n "8.0" 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?\fIvarName\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 \fIvarName\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 \fIvarName\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 If \fIscript\fR does not raise an error, \fBcatch\fR will return 0 (\fBTCL_OK\fR) and set the variable to the value returned from \fIscript\fR. .PP Note that \fBcatch\fR catches all exceptions, including those generated by \fBbreak\fR and \fBcontinue\fR as well as errors. The only errors that are not caught are syntax errors found when the script is compiled. This is because the catch command only catches errors during runtime. When the catch statement is compiled, the script is compiled as well and any syntax errors will generate a Tcl error. .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 $someFile for writing\\n$fid" exit 1 } .CE .PP The \fBcatch\fR command will not catch compiled syntax errors. The first time proc \fBfoo\fR is called, the body will be compiled and a Tcl error will be generated. .CS proc foo {} { \fBcatch\fR {expr {1 +- }} } .CE .SH "SEE ALSO" break(n), continue(n), error(n), return(n), tclvars(n) .SH KEYWORDS catch, error