51 lines
2.4 KiB
HTML
51 lines
2.4 KiB
HTML
<HTML><HEAD><TITLE>Tcl Built-In Commands - catch manual page</TITLE></HEAD><BODY>
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<H3><A NAME="M2">NAME</A></H3>
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catch - Evaluate script and trap exceptional returns
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<H3><A NAME="M3">SYNOPSIS</A></H3>
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<B>catch</B><I> script </I>?<I>varName</I>?<BR>
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<H3><A NAME="M4">DESCRIPTION</A></H3>
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The <B>catch</B> command may be used to prevent errors from aborting command
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interpretation. <B>Catch</B> calls the Tcl interpreter recursively to
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execute <I>script</I>, and always returns without raising an error,
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regardless of any errors that might occur while executing <I>script</I>.
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<P>
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If <I>script</I> raises an error, <B>catch</B> will return a non-zero integer
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value corresponding to one of the exceptional return codes (see tcl.h
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for the definitions of code values). If the <I>varName</I> argument is
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given, then the variable it names is set to the error message from
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interpreting <I>script</I>.
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<P>
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If <I>script</I> does not raise an error, <B>catch</B> will return 0
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(TCL_OK) and set the variable to the value returned from <I>script</I>.
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<P>
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Note that <B>catch</B> catches all exceptions, including those
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generated by <B><A HREF="../TkCmd/break.htm">break</A></B> and <B><A HREF="../TkCmd/continue.htm">continue</A></B> as well as errors. The
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only errors that are not caught are syntax errors found when the
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script is compiled. This is because the catch command only catches
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errors during runtime. When the catch statement is compiled, the
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script is compiled as well and any syntax errors will generate a Tcl
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error.
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<H3><A NAME="M5">EXAMPLES</A></H3>
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The <B>catch</B> command may be used in an <B><A HREF="../TkCmd/if.htm">if</A></B> to branch based on
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the success of a script.
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<PRE>if { [catch {open $someFile w} fid] } {
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puts stderr "Could not open $someFile for writing\n$fid"
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exit 1
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}</PRE>
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The <B>catch</B> command will not catch compiled syntax errors. The
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first time proc <B>foo</B> is called, the body will be compiled and a
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Tcl error will be generated.
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<PRE>proc foo {} {
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catch {expr {1 +- }}
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}</PRE>
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<H3><A NAME="M6">KEYWORDS</A></H3>
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<A href="../Keywords/C.htm#catch">catch</A>, <A href="../Keywords/E.htm#error">error</A>
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<HR><PRE>
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<A HREF="../copyright.htm">Copyright</A> © 1993-1994 The Regents of the University of California.
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<A HREF="../copyright.htm">Copyright</A> © 1994-1996 Sun Microsystems, Inc.
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<A HREF="../copyright.htm">Copyright</A> © 1995-1997 Roger E. Critchlow Jr.</PRE>
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</BODY></HTML>
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