Tk_Preserve

Tk_Preserve(clientData)

Tk_Release(clientData)

Tk_EventuallyFree(clientData, freeProc)

ARGUMENTS

ClientData clientData (in)

Token describing structure to be freed or reallocated. Usually a pointer
to memory for structure.

Tk_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. 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
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.

Tk_Preserve and Tk_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 Tk_Preserve for the object has been
matched by calls to Tk_Release.
There may be any number of unmatched Tk_Preserve calls
in effect at once.

Tk_EventuallyFree is invoked to free up its clientData
argument.
It checks to see if there are unmatched Tk_Preserve calls
for the object.
If not, then Tk_EventuallyFree calls freeProc immediately.
Otherwise Tk_EventuallyFree records the fact that clientData
needs eventually to be freed.
When all calls to Tk_Preserve have been matched with
calls to Tk_Release then freeProc will be called by
Tk_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 Tk_FreeProc:

typedef void Tk_FreeProc(ClientData clientData);

This mechanism can be used to solve the problem described above by placing Tk_Preserve and Tk_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