Differences between static and visibility hidden

People who do C know that if the defined function is only used in this file, it should be declared as static

But looking at the bionic code in Android, bionic/libc/bionic/pthread. c found such code snippets.

   1269 __LIBC_HIDDEN__   1270 int pthread_mutex_unlock_impl(pthread_mutex_t *mutex)   1271 {   1272     int mvalue, mtype, tid, oldv, shared;   1273    1274     if (__unlikely(mutex == NULL))   1275         return EINVAL;   1276    1277     mvalue = mutex->value;   1278     mtype  = (mvalue & MUTEX_TYPE_MASK);   1279     shared = (mvalue & MUTEX_SHARED_MASK);

Since this function is used by other code in this file, why not define it as static?

    #define __LIBC_HIDDEN__\    __attribute__ ((visibility ("hidden")))

According to the information, the original hidden function is to make the function invisible outside the share libs, that is, there should be other files (units) in the library to use this function.

Search, and found

Pthread_debug.c 451 extern int pthread_mutex_unlock_impl (pthread_mutex_t * mutex );

                        458 return pthread_mutex_unlock_impl(mutex);pthread.c        1270 int pthread_mutex_unlock_impl(pthread_mutex_t *mutex) function