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Tcl_TraceCommand(3)	    Tcl	Library	Procedures	   Tcl_TraceCommand(3)


       Tcl_CommandTraceInfo,  Tcl_TraceCommand,	 Tcl_UntraceCommand  - monitor
       renames and deletes of a	command

       #include	<tcl.h>

       Tcl_CommandTraceInfo(interp, cmdName, flags, proc, prevClientData)

       Tcl_TraceCommand(interp,	cmdName, flags,	proc, clientData)

       Tcl_UntraceCommand(interp, cmdName, flags, proc,	clientData)

       Tcl_Interp *interp (in)				 Interpreter  contain-
							 ing the command.

       const char *cmdName (in)				 Name of command.

       int flags (in)					 OR'ed	collection  of
							 the		values
							 TCL_TRACE_RENAME  and

       Tcl_CommandTraceProc *proc (in)			 Procedure   to	  call
							 when specified	opera-
							 tions occur  to  cmd-

       ClientData clientData (in)			 Arbitrary argument to
							 pass to proc.

       ClientData prevClientData (in)			 If  non-NULL,	 gives
							 last  value  returned
							 by  Tcl_CommandTrace-
							 Info,	so  this  call
							 will return  informa-
							 tion	 about	  next
							 trace.	 If NULL, this
							 call  will return in-
							 formation about first

       Tcl_TraceCommand	 allows	 a C procedure to monitor operations performed
       on a Tcl	command, so that the C procedure is invoked whenever the  com-
       mand  is	renamed	or deleted.  If	the trace is created successfully then
       Tcl_TraceCommand	returns	TCL_OK.	If an  error  occurred	(e.g.  cmdName
       specifies a non-existent	command) then TCL_ERROR	is returned and	an er-
       ror message is left in the interpreter's	result.

       The flags argument to Tcl_TraceCommand indicates	when the trace	proce-
       dure  is	 to be invoked.	 It consists of	an OR'ed combination of	any of
       the following values:

	      Invoke proc whenever the command is renamed.

	      Invoke proc when the command is deleted.

       Whenever	one of the specified operations	occurs to  the	command,  proc
       will  be	 invoked.   It should have arguments and result	that match the
       type Tcl_CommandTraceProc:
	      typedef void Tcl_CommandTraceProc(
		      ClientData clientData,
		      Tcl_Interp *interp,
		      const char *oldName,
		      const char *newName,
		      int flags);
       The clientData and interp parameters will have the same values as those
       passed to Tcl_TraceCommand when the trace was created.  ClientData typ-
       ically points to	an application-specific	data structure that  describes
       what to do when proc is invoked.	 OldName gives the name	of the command
       being renamed, and newName gives	the name that the command is being re-
       named  to  (or  NULL  when  the command is being	deleted.)  Flags is an
       OR'ed combination of bits potentially providing several pieces  of  in-
       formation.   One	of the bits TCL_TRACE_RENAME and TCL_TRACE_DELETE will
       be set in flags to indicate which operation is being performed  on  the
       command.	 The bit TCL_TRACE_DESTROYED will be set in flags if the trace
       is about	to be destroyed; this information may be  useful  to  proc  so
       that  it	can clean up its own internal data structures (see the section
       TCL_TRACE_DESTROYED below for more details).  Because the  deletion  of
       commands	can take place as part of the deletion of the interp that con-
       tains them, proc	must be	careful	about checking what the	passed in  in-
       terp  value can be called upon to do.  The routine Tcl_InterpDeleted is
       an important tool for this.  When  Tcl_InterpDeleted  returns  1,  proc
       will not	be able	to invoke any scripts in interp.  The function of proc
       in that circumstance is limited to the cleanup of its own  data	struc-

       Tcl_UntraceCommand may be used to remove	a trace.  If the command spec-
       ified by	interp,	cmdName, and flags has a trace set with	 flags,	 proc,
       and  clientData,	 then  the corresponding trace is removed.  If no such
       trace exists, then the call to Tcl_UntraceCommand has no	 effect.   The
       same bits are valid for flags as	for calls to Tcl_TraceCommand.

       Tcl_CommandTraceInfo  may  be used to retrieve information about	traces
       set on a	given command.	The return value from Tcl_CommandTraceInfo  is
       the  clientData	associated with	a particular trace.  The trace must be
       on the command specified	by the interp, cmdName,	 and  flags  arguments
       (note  that  currently  the flags are ignored; flags should be set to 0
       for future compatibility) and its trace procedure must the same as  the
       proc  argument.	If the prevClientData argument is NULL then the	return
       value corresponds to the	first (most recently created) matching	trace,
       or  NULL	 if there are no matching traces.  If the prevClientData argu-
       ment is not NULL, then it should	be the return value  from  a  previous
       call  to	Tcl_CommandTraceInfo.  In this case, the new return value will
       correspond to the next matching trace after the	one  whose  clientData
       matches	prevClientData,	 or NULL if no trace matches prevClientData or
       if there	are no more matching traces after it.  This mechanism makes it
       possible	 to  step  through  all	of the traces for a given command that
       have the	same proc.

       During rename traces, the command being renamed is  visible  with  both
       names  simultaneously,  and  the	 command  still	 exists	 during	delete
       traces, unless the interp that contains it is being deleted.   However,
       there  is  no mechanism for signaling that an error occurred in a trace
       procedure, so great care	 should	 be  taken  that  errors  do  not  get
       silently	lost.

       It  is possible for multiple traces to exist on the same	command.  When
       this happens, all of the	trace procedures will be invoked on  each  ac-
       cess,  in  order	 from most-recently-created to least-recently-created.
       Attempts	to  delete  the	 command  during  a  delete  trace  will  fail
       silently,  since	 the command is	already	scheduled for deletion anyway.
       If the command being renamed is renamed by one of  its  rename  traces,
       that  renaming  takes  precedence over the one that triggered the trace
       and the collection of traces will not be	reexecuted; if several	traces
       rename the command, the last renaming takes precedence.

       In  a  delete  callback	to proc, the TCL_TRACE_DESTROYED bit is	set in

       clientData, trace, command

Tcl				      7.4		   Tcl_TraceCommand(3)


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