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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.
'\"
'\" RCS: @(#) $Id: error.n,v 1.12 2008/10/15 10:43:37 dkf Exp $
'\"
.so man.macros
.TH error n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note: do not modify the .SH NAME line immediately below!
.SH NAME
error \- Generate an error
.SH SYNOPSIS
\fBerror \fImessage\fR ?\fIinfo\fR? ?\fIcode\fR?
.BE
.SH DESCRIPTION
.PP
Returns a \fBTCL_ERROR\fR code, which causes command interpretation to be
unwound. \fIMessage\fR is a string that is returned to the application
to indicate what went wrong.
.PP
The \fB\-errorinfo\fR return option of an interpreter is used
to accumulate a stack trace of what was in progress when an
error occurred; as nested commands unwind,
the Tcl interpreter adds information to the \fB\-errorinfo\fR
return option. If the \fIinfo\fR argument is present, it is
used to initialize the \fB\-errorinfo\fR return options and
the first increment of unwind information
will not be added by the Tcl interpreter.
In other
words, the command containing the \fBerror\fR command will not appear
in the stack trace; in its place will be \fIinfo\fR.
Historically, this feature had been most useful in conjunction
with the \fBcatch\fR command:
if a caught error cannot be handled successfully, \fIinfo\fR can be used
to return a stack trace reflecting the original point of occurrence
of the error:
.PP
.CS
\fBcatch {...} errMsg
set savedInfo $::errorInfo
\&...
error $errMsg $savedInfo\fR
.CE
.PP
When working with Tcl 8.5 or later, the following code
should be used instead:
.PP
.CS
\fBcatch {...} errMsg options
\&...
return -options $options $errMsg\fR
.CE
.PP
If the \fIcode\fR argument is present, then its value is stored
in the \fB\-errorcode\fR return option. The \fB\-errorcode\fR
return option is intended to hold a machine-readable description
of the error in cases where such information is available; see
the \fBreturn\fR manual page for information on the proper format
for this option's value.
.SH EXAMPLE
Generate an error if a basic mathematical operation fails:
.CS
if {1+2 != 3} {
\fBerror\fR "something is very wrong with addition"
}
.CE
.SH "SEE ALSO"
catch(n), return(n)
.SH KEYWORDS
error
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