- 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
- DESCRIPTION
- SEE
ALSO
- KEYWORDS
Tcl_Preserve, Tcl_Release, Tcl_EventuallyFree — avoid freeing
storage while it is being used
#include <tcl.h>
Tcl_Preserve(clientData)
Tcl_Release(clientData)
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 clientData.
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 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 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.
Tcl_Interp, Tcl_Alloc
free, reference count, storage
Copyright © 1990 The Regents of the University of
California.
Copyright © 1994-1996 Sun Microsystems, Inc.