TclTraceCommand(3) Tcl Library Procedures TclTraceCommand(3)

NAME
       TclCommandTraceInfo,  TclTraceCommand,  TclUntraceCommand  - monitor
       renames and deletes of a command

SYNOPSIS
       ##iinncclluuddee <>

       ClientData
       TTccllCCoommmmaannddTTrraacceeIInnffoo((interp, cmdName, flags, proc, prevClientData))

       int
       TTccllTTrraacceeCCoommmmaanndd((interp, cmdName, flags, proc, clientData))

       void
       TTccllUUnnttrraacceeCCoommmmaanndd((interp, cmdName, flags, proc, clientData))

AARRGGUUMMEENNTTSS
       TclInterp             *interp          (in)      Interpreter  contain-
                                                         ing the command.

       CONST char             *cmdName         (in)      Name of command.

       int                    flags            (in)      OR-ed  collection  of
                                                         the             value
                                                         TCLTRACERENAME  and
                                                         TCLTRACEDELETE.

       TclCommandTraceProc   *proc            (in)      Procedure   to   call
                                                         when specified opera-
                                                         tions occur  to  cmd-
                                                         Name.

       ClientData             clientData       (in)      Arbitrary argument to
                                                         pass to proc.

       ClientData             prevClientData   (in)      If  non-NULL,   gives
                                                         last  value  returned
                                                         by  TTccllCCoommmmaannddTTrraaccee-
                                                         IInnffoo,  so  this  call
                                                         will return  informa-
                                                         tion    about    next
                                                         trace.  If NULL, this
                                                         call    will   return
                                                         information     about
                                                         first trace.



DESCRIPTION
       TTccllTTrraacceeCCoommmmaanndd  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
       TTccllTTrraacceeCCoommmmaanndd returns TCLOK.  If an error  occurred  (e.g.  cmdName
       specifies  a  non-existent  command)  then TCLERROR is returned and an
       error message is left in the interpreter's result.

       The flags argument to TTccllTTrraacceeCCoommmmaanndd indicates when the trace  proce-
       dure  is  to be invoked.  It consists of an OR-ed combination of any of
       the following values:

       TCLTRACERENAME
              Invoke proc whenever the command is renamed.

       TTCCLLTTRRAACCEEDDEELLEETTEE
              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 TTccllCCoommmmaannddTTrraacceePPrroocc:
              typedef void TclCommandTraceProc(
                ClientData clientData,
                      TclInterp *interp,
                CONST char *oldName,
                CONST char *newName,
                      int flags);
       The clientData and interp parameters will have the same values as those
       passed to TTccllTTrraacceeCCoommmmaanndd 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 TCLTRACERENAME and
       TCLTRACEDELETE will be set in flags to indicate  which  operation  is
       being  performed  on  the command.  The bit TCLTRACEDESTROYED 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  TCLTRACEDESTROYED  below   for   more
       details).   Lastly,  the  bit  TCLINTERPDESTROYED  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
       TCLINTERPDESTROYED below).

       TTccllUUnnttrraacceeCCoommmmaanndd 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 TTccllUUnnttrraacceeCCoommmmaanndd has no effect.  The
       same bits are valid for flags as for calls to TTccllTTrraacceeCCoommmmaanndd.

       TTccllCCoommmmaannddTTrraacceeIInnffoo may be used to retrieve information  about  traces
       set  on a given command.  The return value from TTccllCCoommmmaannddTTrraacceeIInnffoo 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 TTccllCCoommmmaannddTTrraacceeIInnffoo.  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.


CCAALLLLIINNGG CCOOMMMMAANNDDSS DDUURRIINNGG TTRRAACCEESS
       During  rename  traces,  the command being renamed is visible with both
       names simultaneously, and the command still exists during delete traces
       (if  TCLINTERPDESTROYED  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.


MMUULLTTIIPPLLEE TTRRAACCEESS
       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.


TTCCLLTTRRAACCEEDDEESSTTRROOYYEEDD FFLLAAGG
       In  a  delete  callback  to proc, the TCLTRACEDESTROYED bit is set in
       flags.



TTCCLLIINNTTEERRPPDDEESSTTRROOYYEEDD
       When an interpreter is destroyed, unset traces are called  for  all  of
       its  commands.   The  TCLINTERPDESTROYED 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 TCLINTERPDESTROYED 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 mis-
       using the interpreter during traces with TCLINTERPDESTROYED set.


KKEEYYWWOORRDDSS
       clientData, trace, command



Tcl                                   7.4                  TclTraceCommand(3)