Provided by: tcl8.6-doc_8.6.12+dfsg-1build1_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