问题
I am trying to understand the purpose of the definition of atomic_forced_read which shows up frequently in the GNU libc implementation of malloc.c.
I am not great when it comes to inline assembly, but it looks like this returns the exact same value, with the same type as the input value. what am I missing here?
Atomic forced read definition in atomic.h
523 #ifndef atomic_forced_read
524 # define atomic_forced_read(x) \
525 ({ __typeof (x) __x; __asm ("" : "=r" (__x) : "0" (x)); __x; })
526 #endif
Link to atomic.h
https://code.woboq.org/userspace/glibc/include/atomic.h.html
回答1:
One usage is of atomic_forced_read:
#if HAVE_MALLOC_INIT_HOOK
void (*hook) (void) = atomic_forced_read(__malloc_initialize_hook);
if (hook != NULL)
(*hook)();
#endif
It appears that __malloc_initialize_hook can be changed from another thread, so that if __malloc_initialize_hook is loaded once more from memory after the NULL check its value may have changed back to NULL.
atomic_forced_read makes sure that __malloc_initialize_hook is loaded into a register due to =routput constraint, so that __malloc_initialize_hook isn't reloaded from memory after the NULL check. That empty asm breaks compiler dependency of hook on __malloc_initialize_hook, since hook is now initialized with __x stored in a register and not __malloc_initialize_hook. After hook has been initialized with __x, the latter is gone and cannot be possibly reloaded.
In C11 mode that __malloc_initialize_hook can be atomic_uintptr_t and atomic_load_explicit(&__malloc_initialize_hook, memory_order_relaxed) can be used instead of atomic_forced_read to load __malloc_initialize_hook from memory exactly once.
来源：https://stackoverflow.com/questions/58082597/what-is-the-purpose-of-glibcs-atomic-forced-read-function