Tcl_Preserve,   Tcl_Release,  Tcl_EventuallyFree  -  avoid
       freeing storage while it's being used


       #include <tcl.h>



       Tcl_EventuallyFree(clientData, freeProc)


       ClientData     clientData   (in)      Token     describing
                                             structure    to   be
                                             freed   or   reallo­
                                             cated.    Usually  a
                                             pointer  to   memory
                                             for structure.

       Tcl_FreeProc   *freeProc    (in)      Procedure  to invoke
                                             to free  clientData.


       These  three  procedures help implement a simple reference
       count mechanism for managing storage.  They  are  designed
       to  solve a problem having to do with widget deletion, but
       are also useful in many other situations.  When  a  widget
       is  deleted,  its  widget  record  (the  structure holding
       information specific to the widget) must  be  returned  to
       the  storage  allocator.   However, it's possible that the
       widget record is in active use by one of the procedures on
       the  stack  at the time of the deletion.  This can happen,
       for example, if the command associated with a button  wid­
       get  causes the button to be destroyed:  an X event causes
       an event-handling C procedure in the button to be invoked,
       which  in  turn causes the button's associated Tcl command
       to be executed, which in turn  causes  the  button  to  be
       deleted,  which  in turn causes the button's widget record
       to be de-allocated.  Unfortunately, when the  Tcl  command
       returns,  the  button's event-handling procedure will need
       to reference the button's widget record.  Because of this,
       the  widget  record must not be freed as part of the dele­
       tion, but must be retained until the event-handling proce­
       dure  has finished with it.  In other situations where the
       widget is deleted, it may be possible to free  the  widget
       record immediately.

       clientData  argument identifies an object and usually con­
       sists of the address of a structure.  The reference counts
       guarantee that an object will not be freed until each call
       to Tcl_Preserve for the object has been matched  by  calls
       to  Tcl_Release.   There  may  be  any number of unmatched
       Tcl_Preserve calls in effect at once.

       Tcl_EventuallyFree is invoked to free  up  its  clientData
       argument.   It  checks  to  see  if  there  are  unmatched
       Tcl_Preserve calls for the object.  If not, then Tcl_Even­
       tuallyFree    calls   freeProc   immediately.    Otherwise
       Tcl_EventuallyFree records the fact that clientData  needs
       eventually  to  be  freed.  When all calls to Tcl_Preserve
       have been matched with calls to Tcl_Release then  freeProc
       will be called by Tcl_Release to do the cleanup.

       All  the  work  of  freeing  the  object is carried out by
       freeProc.  FreeProc must have arguments  and  result  that
       match the type Tcl_FreeProc:
              typedef void Tcl_FreeProc(char *blockPtr);
       The  blockPtr argument to freeProc will be the same as the
       clientData argument to Tcl_EventuallyFree.   The  type  of
       blockPtr  (char  *)  is  different  than  the  type of the
       clientData argument to Tcl_EventuallyFree  for  historical
       reasons, but the value is the same.

       This  mechanism can be used to solve the problem described
       above by placing Tcl_Preserve and Tcl_Release calls around
       actions  that  may  cause undesired storage re-allocation.
       The mechanism is intended only for  short-term  use  (i.e.
       while  procedures  are pending on the stack);  it will not
       work efficiently as a mechanism  for  long-term  reference
       counts.   The implementation does not depend in any way on
       the internal structure of the  objects  being  freed;   it
       keeps the reference counts in a separate structure.


       free, reference count, storage