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 buttons associated Tcl command to be executed,
which in turn causes the button to be deleted, which in turn causes
the buttons widget record to be de-allocated.
Unfortunately, when the Tcl command returns, the buttons
event-handling procedure will need to reference the
buttons 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
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
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
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.
typedef void Tcl_FreeProc(char *blockPtr);
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
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.