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<HTML><HEAD><TITLE>Library Procedures - Tcl_CreateTrace manual page</TITLE></HEAD><BODY>
<H3><A NAME="M2">NAME</A></H3>
Tcl_CreateTrace, Tcl_DeleteTrace - arrange for command execution to be traced
<H3><A NAME="M3">SYNOPSIS</A></H3>
<B>#include &lt;tcl.h&gt;</B><BR>
Tcl_Trace<BR>
<B>Tcl_CreateTrace</B>(<I>interp, level, proc, clientData</I>)<BR>
<B>Tcl_DeleteTrace</B>(<I>interp, trace</I>)<BR>
<H3><A NAME="M4">ARGUMENTS</A></H3>
<DL>
<P><DT><A HREF="../TclLib/Interp.htm">Tcl_Interp</A> <B>*interp</B> (in)<DD>
Interpreter containing command to be traced or untraced.
<P><DT>int <B>level</B> (in)<DD>
Only commands at or below this nesting level will be traced.  1 means
top-level commands only, 2 means top-level commands or those that are
invoked as immediate consequences of executing top-level commands
(procedure bodies, bracketed commands, etc.) and so on.
<P><DT>Tcl_CmdTraceProc <B>*proc</B> (in)<DD>
Procedure to call for each command that's executed.  See below for
details on the calling sequence.
<P><DT>ClientData <B>clientData</B> (in)<DD>
Arbitrary one-word value to pass to <I>proc</I>.
<P><DT>Tcl_Trace <B><A HREF="../TclCmd/trace.htm">trace</A></B> (in)<DD>
Token for trace to be removed (return value from previous call
to <B>Tcl_CreateTrace</B>).
<P></DL>
<H3><A NAME="M5">DESCRIPTION</A></H3>
<B>Tcl_CreateTrace</B> arranges for command tracing.  From now on, <I>proc</I>
will be invoked before Tcl calls command procedures to process
commands in <I>interp</I>.  The return value from
<B>Tcl_CreateTrace</B> is a token for the trace,
which may be passed to <B>Tcl_DeleteTrace</B> to remove the trace.  There may
be many traces in effect simultaneously for the same command interpreter.
<P>
<I>Proc</I> should have arguments and result that match the
type <B>Tcl_CmdTraceProc</B>:
<PRE>typedef void Tcl_CmdTraceProc(
	ClientData <I>clientData</I>,
	<A HREF="../TclLib/Interp.htm">Tcl_Interp</A> *<I>interp</I>,
	int <I>level</I>,
	char *<I>command</I>,
	Tcl_CmdProc *<I>cmdProc</I>,
	ClientData <I>cmdClientData</I>,
	int <I>argc</I>,
	char *<I>argv</I>[]);</PRE>
The <I>clientData</I> and <I>interp</I> parameters are
copies of the corresponding arguments given to <B>Tcl_CreateTrace</B>.
<I>ClientData</I> typically points to an application-specific
data structure that describes what to do when <I>proc</I>
is invoked.  <I>Level</I> gives the nesting level of the command
(1 for top-level commands passed to <B><A HREF="../TclLib/Eval.htm">Tcl_Eval</A></B> by the application,
2 for the next-level commands passed to <B><A HREF="../TclLib/Eval.htm">Tcl_Eval</A></B> as part of parsing
or interpreting level-1 commands, and so on).  <I>Command</I>
points to a string containing the text of the
command, before any argument substitution.
<I>CmdProc</I> contains the address of the command procedure that
will be called to process the command (i.e. the <I>proc</I> argument
of some previous call to <B><A HREF="../TclLib/CrtCommand.htm">Tcl_CreateCommand</A></B>) and <I>cmdClientData</I>
contains the associated client data for <I>cmdProc</I> (the <I>clientData</I>
value passed to <B><A HREF="../TclLib/CrtCommand.htm">Tcl_CreateCommand</A></B>).  <I>Argc</I> and <I>argv</I> give
the final argument information that will be passed to <I>cmdProc</I>, after
command, variable, and backslash substitution.
<I>Proc</I> must not modify the <I>command</I> or <I>argv</I> strings.
<P>
Tracing will only occur for commands at nesting level less than
or equal to the <I>level</I> parameter (i.e. the <I>level</I>
parameter to <I>proc</I> will always be less than or equal to the
<I>level</I> parameter to <B>Tcl_CreateTrace</B>).
<P>
Calls to <I>proc</I> will be made by the Tcl parser immediately before
it calls the command procedure for the command (<I>cmdProc</I>).  This
occurs after argument parsing and substitution, so tracing for
substituted commands occurs before tracing of the commands
containing the substitutions.  If there is a syntax error in a
command, or if there is no command procedure associated with a
command name, then no tracing will occur for that command.  If a
string passed to <A HREF="../TclLib/Eval.htm">Tcl_Eval</A> contains multiple commands (bracketed, or
on different lines) then multiple calls to <I>proc</I> will occur,
one for each command.  The <I>command</I> string for each of these
trace calls will reflect only a single command, not the entire string
passed to <A HREF="../TclLib/Eval.htm">Tcl_Eval</A>.
<P>
<B>Tcl_DeleteTrace</B> removes a trace, so that no future calls will be
made to the procedure associated with the trace.  After <B>Tcl_DeleteTrace</B>
returns, the caller should never again use the <I>trace</I> token.

<H3><A NAME="M6">KEYWORDS</A></H3>
<A href="../Keywords/C.htm#command">command</A>, <A href="../Keywords/C.htm#create">create</A>, <A href="../Keywords/D.htm#delete">delete</A>, <A href="../Keywords/I.htm#interpreter">interpreter</A>, <A href="../Keywords/T.htm#trace">trace</A>
<HR><PRE>
<A HREF="../copyright.htm">Copyright</A> &#169; 1989-1993 The Regents of the University of California.
<A HREF="../copyright.htm">Copyright</A> &#169; 1994-1996 Sun Microsystems, Inc.
<A HREF="../copyright.htm">Copyright</A> &#169; 1995-1997 Roger E. Critchlow Jr.</PRE>
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