Provided by: tcl8.4-doc_8.4.20-8_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 value
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 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 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.
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.
TCL_INTERP_DESTROYED
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.
BUGS
Tcl doesn't do any error checking to prevent trace procedures from misusing the
interpreter during traces with TCL_INTERP_DESTROYED set.
KEYWORDS
clientData, trace, command