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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  reallo-
					     cated.  Usually a pointer to memory for structure.

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

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.	 Unfortu-
       nately,	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 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 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 argu-
       ments 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_Even-
       tuallyFree.   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.

       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.

SEE ALSO
       Tcl_Interp, Tcl_Alloc

KEYWORDS
       free, reference count, storage

Tcl					       7.5				  Tcl_Preserve(3)
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