TclPreserve(3) Tcl Library Procedures TclPreserve(3)

NAME
       TclPreserve,  TclRelease,  TclEventuallyFree - avoid freeing storage
       while it's being used

SYNOPSIS
       ##iinncclluuddee <>

       TTccllPPrreesseerrvvee(clientData)

       TTccllRReelleeaassee(clientData)

       TTccllEEvveennttuuaallllyyFFrreeee(clientData, freeProc)

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

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



DESCRIPTION
       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.

       TTccllPPrreesseerrvvee and TTccllRReelleeaassee 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  TTccllPPrreesseerrvvee  for  the  object  has  been  matched  by   calls   to
       TTccllRReelleeaassee.   There  may be any number of unmatched TTccllPPrreesseerrvvee calls
       in effect at once.

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

       All the work of freeing the object is carried out by freeProc.   FreeP-
       roc must have arguments and result that match the type TTccllFFrreeeePPrroocc:
              typedef void TclFreeProc(char *blockPtr);
       The  blockPtr  argument  to freeProc will be the same as the clientData
       argument to TTccllEEvveennttuuaallllyyFFrreeee.  The type of blockPtr (cchhaarr **) is  dif-
       ferent  than  the type of the clientData argument to TTccllEEvveennttuuaallllyyFFrreeee
       for historical reasons, but the value is the same.

       When the clientData argument to TTccllEEvveennttuuaallllyyFFrreeee  refers  to  storage
       allocated  and  returned  by  a  prior  call  to TTccllAAlllloocc, cckkaalllloocc, or
       another function of the Tcl library, then the freeProc argument  should
       be given the special value of TTCCLLDDYYNNAAMMIICC.

       This  mechanism  can  be  used  to solve the problem described above by
       placing TTccllPPrreesseerrvvee and TTccllRReelleeaassee  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.


SEE ALSO
       TclInterp, TclAlloc


KKEEYYWWOORRDDSS
       free, reference count, storage



Tcl                                   7.5                      TclPreserve(3)