Summary of cleanup rules

This document states general rules for cleanup.

The general rules for cleanup during the lifetime of an object are:

• between the moment the object is allocated, and the moment that a pointer to that object is stored in a structure which would be accessible if a leave occurred, the object must be pushed to the clean-up stack if a leave could possibly occur

• when a pointer to the object has been stored in an object which would be accessible if a leave occurred, then the object should be popped from the cleanup stack before a leave could possibly occur: normally, the object is popped first, and then the pointer is stored

• an object is accessible if a leave occurs, if either it is pushed to the cleanup stack, or there is a reference to it from another object which would be accessible if a leave occurred, or there is a reference to it from a stack frame which would not be affected when a leave occurs (because of an appropriately-placed TRAP), or there is a reference to it from a global variable

• it should never be possible to clean up an object more than once, if a leave occurred

In practice, this means that an object must be referred to either by a cleanup stack frame, or by a reference from another object. More rarely, the object may be referred to by a stack frame which cannot be unwound during the lifetime of that object, because of an appropriate TRAP. More rarely still, the object may be referred to by a global variable.