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

Tcl                                                    7.4                                Tcl_TraceCommand(3tcl)