Tcl_Preserve(3) Tcl Library Procedures Tcl_Preserve(3)_________________________________________________________________NAME
Tcl_Preserve, Tcl_Release, Tcl_EventuallyFree - avoid
freeing storage while it's 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'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 wid-
get 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 dele-
tion, but must be retained until the event-handling proce-
dure 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
Tcl 7.5 1
Tcl_Preserve(3) Tcl Library Procedures Tcl_Preserve(3)
reference counts for their clientData argument. The
clientData argument identifies an object and usually con-
sists 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_Even-
tuallyFree 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.
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.
KEYWORDS
free, reference count, storage
Tcl 7.5 2
