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Tcl_Preserve(3)                            Tcl Library Procedures                            Tcl_Preserve(3)



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NAME
       Tcl_Preserve, Tcl_Release, Tcl_EventuallyFree - avoid freeing storage while it is 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.  Usually
                                             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 information  specific
       to  the  widget)  must be returned to the storage allocator.  However, it is 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 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 deletion,  but  must  be
       retained until the event-handling procedure has finished with it.  In other situations where the wid-get widget
       get 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
       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 immedi-ately. immediately.
       ately.  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_Eventual-lyFree Tcl_EventuallyFree
       lyFree 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 sepa-rate separate
       rate structure.


SEE ALSO
       Tcl_Interp, Tcl_Alloc


KEYWORDS
       free, reference count, storage



Tcl                                                  7.5                                     Tcl_Preserve(3)