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example:
#define AALLOC_THREAD_ENTER(ap) @synchronized(aalloc_guard) {
#define AALLOC_THREAD_LEAVE(ap) }
notes:
• you must declare and define eventual guard objects yourself
• while there will not be any direct “return” calls inside the
locked area, AALLOC_ABORT and AALLOC_WARN may be called
• AALLOC_RANDOM does not need to be thread-safe, but malloc
and free should be; if “ap” is used for locking, all functions
called must be thread-safe though