Tcl_TraceCommand(3) Tcl Library Procedures Tcl_TraceCommand(3)______________________________________________________________________________NAME
Tcl_CommandTraceInfo, Tcl_TraceCommand, Tcl_UntraceCommand - monitor
renames and deletes of a command
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
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
Info, so this call
will return informa‐
tion about next
trace. If NULL, this
call will return
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
error 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
typedef void Tcl_CommandTraceProc(
CONST char *oldName,
CONST char *newName,
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
renamed to (or an empty string 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). Lastly, the bit TCL_INTERP_DESTROYED will be set if the
entire interpreter is being destroyed. When this bit is set, proc must
be especially careful in the things it does (see the section
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 isn't 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
(if TCL_INTERP_DESTROYED is not set). 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
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.
In a delete callback to proc, the TCL_TRACE_DESTROYED bit is set in
When an interpreter is destroyed, unset traces are called for all of
its commands. The TCL_INTERP_DESTROYED bit will be set in the flags
argument passed to the trace procedures. Trace procedures must be
extremely careful in what they do if the TCL_INTERP_DESTROYED bit is
set. It is not safe for the procedures to invoke any Tcl procedures on
the interpreter, since its state is partially deleted. All that trace
procedures should do under these circumstances is to clean up and free
their own internal data structures.
Tcl doesn't do any error checking to prevent trace procedures from mis‐
using the interpreter during traces with TCL_INTERP_DESTROYED set.
clientData, trace, command
Tcl 7.4 Tcl_TraceCommand(3)