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sem_getvalue - get the value of a semaphore
int sem_getvalue(sem_t *sem, int *sval);
sem_getvalue() places the current value of the semaphore pointed to sem
into the integer pointed to by sval.
If one or more processes or threads are blocked waiting to lock the
semaphore with sem_wait(3), POSIX.1-2001 permits two possibilities for
the value returned in sval: either 0 is returned; or a negative number
whose absolute value is the count of the number of processes and
threads currently blocked in sem_wait(3). Linux adopts the former
sem_getvalue() returns 0 on success; on error, -1 is returned and errno
is set to indicate the error.
EINVAL sem is not a valid semaphore.
The value of the semaphore may already have changed by the time
sem_post(3), sem_wait(3), sem_overview(7)
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