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 hold-
       ing  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 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 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.

SEE ALSO

       Tcl_Interp, Tcl_Alloc

KEYWORDS

       free, reference count, storage

Tcl									7.5							   Tcl_Preserve(3)