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 reallocated.  Usually a
                                             pointer to memory for  structure.

       Tcl_FreeProc   *freeProc    (in)      Procedure   to   invoke  to  free


       These three procedures help implement a simple reference  count  mecha-
       nism for managing storage.  They are designed to solve a problem having
       to do with widget deletion, but are also useful in  many  other  situa-
       tions.   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 widget 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 but-
       ton'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 deletion, but must be retained
       until the event-handling procedure has finished with it.  In other sit-
       uations  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  argument  identifies an
       object and usually consists of the address of a structure.  The  refer-
       ence  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

       All the work of freeing the object is carried out by freeProc.   FreeP-
       roc 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  dif-
       ferent  than  the type of the clientData argument to Tcl_EventuallyFree
       for historical reasons, but the value is the same.

       When the clientData argument to Tcl_EventuallyFree  refers  to  storage
       allocated  and  returned  by  a  prior  call  to Tcl_Alloc, ckalloc, or
       another function of the Tcl library, then the freeProc argument  should
       be given the special value of TCL_DYNAMIC.

       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.


       Tcl_Interp, Tcl_Alloc


       free, reference count, storage

Tcl                                   7.5                      Tcl_Preserve(3)

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