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


#include <tcl.h>

interp, cmdName, flags, proc, prevClientData)

interp, cmdName, flags, proc, clientData)

interp, cmdName, flags, proc, clientData)


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.



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:

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 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 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_INTERP_DESTROYED below).

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.


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 flags.


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 does not do any error checking to prevent trace procedures from misusing the interpreter during traces with TCL_INTERP_DESTROYED set.


clientData, trace, command


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