1Tcl_Preserve(3) Tcl Library Procedures Tcl_Preserve(3)
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8 Tcl_Preserve, Tcl_Release, Tcl_EventuallyFree - avoid freeing storage
9 while it is being used
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12 #include <tcl.h>
13 Tcl_Preserve(clientData)
15 Tcl_Release(clientData)
17 Tcl_EventuallyFree(clientData, freeProc)
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21 ClientData clientData (in) Token describing structure to be
22 freed or reallocated. Usually a
23 pointer to memory for structure.
24 Tcl_FreeProc *freeProc (in) Procedure to invoke to free
26 clientData.
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31 These three procedures help implement a simple reference count mecha‐
32 nism for managing storage. They are designed to solve a problem having
33 to do with widget deletion, but are also useful in many other situa‐
34 tions. When a widget is deleted, its widget record (the structure
35 holding information specific to the widget) must be returned to the
36 storage allocator. However, it is possible that the widget record is
37 in active use by one of the procedures on the stack at the time of the
38 deletion. This can happen, for example, if the command associated with
39 a button widget causes the button to be destroyed: an X event causes
40 an event-handling C procedure in the button to be invoked, which in
41 turn causes the button's associated Tcl command to be executed, which
42 in turn causes the button to be deleted, which in turn causes the but‐
43 ton's widget record to be de-allocated. Unfortunately, when the Tcl
44 command returns, the button's event-handling procedure will need to
45 reference the button's widget record. Because of this, the widget
46 record must not be freed as part of the deletion, but must be retained
47 until the event-handling procedure has finished with it. In other sit‐
48 uations where the widget is deleted, it may be possible to free the
49 widget record immediately.
50 Tcl_Preserve and Tcl_Release implement short-term reference counts for
52 their clientData argument. The clientData argument identifies an
53 object and usually consists of the address of a structure. The refer‐
54 ence counts guarantee that an object will not be freed until each call
55 to Tcl_Preserve for the object has been matched by calls to
56 Tcl_Release. There may be any number of unmatched Tcl_Preserve calls
57 in effect at once.
58 Tcl_EventuallyFree is invoked to free up its clientData argument. It
60 checks to see if there are unmatched Tcl_Preserve calls for the object.
61 If not, then Tcl_EventuallyFree calls freeProc immediately. Otherwise
62 Tcl_EventuallyFree records the fact that clientData needs eventually to
63 be freed. When all calls to Tcl_Preserve have been matched with calls
64 to Tcl_Release then freeProc will be called by Tcl_Release to do the
65 cleanup.
66 All the work of freeing the object is carried out by freeProc. FreeP‐
68 roc must have arguments and result that match the type Tcl_FreeProc:
69 typedef void Tcl_FreeProc(char *blockPtr);
70 The blockPtr argument to freeProc will be the same as the clientData
71 argument to Tcl_EventuallyFree. The type of blockPtr (char *) is dif‐
72 ferent than the type of the clientData argument to Tcl_EventuallyFree
73 for historical reasons, but the value is the same.
74 When the clientData argument to Tcl_EventuallyFree refers to storage
76 allocated and returned by a prior call to Tcl_Alloc, ckalloc, or
77 another function of the Tcl library, then the freeProc argument should
78 be given the special value of TCL_DYNAMIC.
79 This mechanism can be used to solve the problem described above by
81 placing Tcl_Preserve and Tcl_Release calls around actions that may
82 cause undesired storage re-allocation. The mechanism is intended only
83 for short-term use (i.e. while procedures are pending on the stack);
84 it will not work efficiently as a mechanism for long-term reference
85 counts. The implementation does not depend in any way on the internal
86 structure of the objects being freed; it keeps the reference counts in
87 a separate structure.
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91 Tcl_Interp, Tcl_Alloc
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95 free, reference count, storage
96Tcl 7.5 Tcl_Preserve(3)