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NAME
     Tcl_Preserve, Tcl_Release, Tcl_EventuallyFree - avoid  free-
     ing storage while it's being used

SYNOPSIS
     #include <tcl.h>

     Tcl_Preserve(clientData)

     Tcl_Release(clientData)

     Tcl_EventuallyFree(clientData, freeProc)

ARGUMENTS
     ClientData     clientData   (in)      Token       describing
                                           structure  to be freed
                                           or reallocated.   Usu-
                                           ally   a   pointer  to
                                           memory for structure.

     Tcl_FreeProc   *freeProc    (in)      Procedure to invoke to
                                           free clientData.
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DESCRIPTION
     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  informa-
     tion 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 widget causes the button to
     be destroyed:  an X event causes an  event-handling  C  pro-
     cedure 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.   Unfor-
     tunately,  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 deletion, but must be retained until
     the event-handling procedure has finished with it.  In other
     situations where the widget is deleted, it may  be  possible
     to free the widget record immediately.


     Tcl_Preserve and Tcl_Release implement short-term  reference
     counts  for their clientData argument.  The clientData argu-
     ment identifies  an  object  and  usually  consists  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_EventuallyFree  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  client-
     Data  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  inter-
     nal  structure  of  the  objects  being freed;  it keeps the
     reference counts in a separate structure.


KEYWORDS
     free, reference count, storage

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