#include <synch.h> (or #include <thread.h>) int mutex_init(mutex_t *mp, int type, void *arg));
When a process dies while holding a USYNC_PROCESS lock, subsequent requestors of that lock hang. This is a problem for systems which share locks with client processes because the client processes can be abnormally killed. To avoid the problem of hanging on a lock held by a dead process, use USYNC_PROCESS_ROBUST to lock the mutex. USYNC_PROCESS_ROBUST adds two capabilities:
In the case of process death, all owned locks held by that process are unlocked.
The next requestor for any of the locks owned by the dead process receives the lock, but with an error return indicating that the previous owner died while holding the lock..
Mutexes can also be initialized by allocation in zeroed memory, in which case a type of USYNC_THREAD is assumed.
Multiple threads must not initialize the same mutex simultaneously. A mutex lock must not be reinitialized while other threads might be using it.
#include <thread.h> mutex_t mp; int ret; /* to be used within this process only */ ret = mutex_init(&mp, USYNC_THREAD, 0);
#include <thread.h> mutex_t mp; int ret; /* to be used among all processes */ ret = mutex_init(&mp, USYNC_PROCESS, 0);
#include <thread.h> mutex_t mp; int ret; /* to be used among all processes */ ret = mutex_init(&mp, USYNC_PROCESS_ROBUST, 0);