#include <semaphore.h> int sem_getvalue(sem_t *restrict sem, int *restrict sval);
The sem_getvalue() function updates the location referenced by the sval argument to have the value of the semaphore referenced by sem without affecting the state of the semaphore. The updated value represents an actual semaphore value that occurred at some unspecified time during the call, but it need not be the actual value of the semaphore when it is returned to the calling process.
If sem is locked, then the value returned by sem_getvalue() is either zero or a negative number whose absolute value represents the number of processes waiting for the semaphore at some unspecified time during the call.
The value set in sval may be 0 or positive. If sval is 0, there may be other processes (or LWPs or threads) waiting for the semaphore; if sval is positive, no process is waiting.
Upon successful completion, sem_getvalue() returns 0. Otherwise, it returns −1 and sets errno to indicate the error.
The sem_getvalue() function will fail if:
The sem argument does not refer to a valid semaphore.
The sem_getvalue() function is not supported by the system.
See attributes(5) for descriptions of the following attributes: