Provided by: tcl8.5-doc_8.5.19-4_all 
NAME
Tcl_CommandTraceInfo, Tcl_TraceCommand, Tcl_UntraceCommand - monitor renames and deletes of a command
SYNOPSIS
#include <tcl.h>
ClientData
Tcl_CommandTraceInfo(interp, cmdName, flags, proc, prevClientData)
int
Tcl_TraceCommand(interp, cmdName, flags, proc, clientData)
void
Tcl_UntraceCommand(interp, cmdName, flags, proc, clientData)
ARGUMENTS
Tcl_Interp *interp (in) Interpreter containing the command.
const char *cmdName (in) Name of command.
int flags (in) OR'ed collection of the values TCL_TRACE_RENAME and
TCL_TRACE_DELETE.
Tcl_CommandTraceProc *proc (in) Procedure to call when specified operations occur to
cmdName.
ClientData clientData (in) Arbitrary argument to pass to proc.
ClientData prevClientData (in) If non-NULL, gives last value returned by
Tcl_CommandTraceInfo, so this call will return
information about next trace. If NULL, this call will
return information about first trace.
________________________________________________________________________________________________________________
DESCRIPTION
Tcl_TraceCommand allows a C procedure to monitor operations performed on a Tcl command, so that the C
procedure is invoked whenever the command 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 error message is left in the interpreter's result.
The flags argument to Tcl_TraceCommand indicates when the trace procedure is to be invoked. It consists
of an OR'ed combination of any of the following values:
TCL_TRACE_RENAME
Invoke proc whenever the command is renamed.
TCL_TRACE_DELETE
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 typically 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 renamed to (or NULL when the command is being deleted.)
Flags is an OR'ed combination of bits potentially providing several pieces of information. 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 contains them, proc must be careful
about checking what the passed in interp 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
structures.
Tcl_UntraceCommand may be used to remove a trace. If the command specified 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
argument 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.
CALLING COMMANDS DURING TRACES
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.
MULTIPLE TRACES
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 access, 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.
TCL_TRACE_DESTROYED FLAG
In a delete callback to proc, the TCL_TRACE_DESTROYED bit is set in flags.
KEYWORDS
clientData, trace, command