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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.
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.
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