sem_getvalue - Man Page

get the value of a semaphore


#include <semaphore.h>

int sem_getvalue(sem_t *restrict sem, int *restrict sval);

Link with -pthread.


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 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 behavior.

Return Value

sem_getvalue() returns 0 on success; on error, -1 is returned and errno is set to indicate the error.



sem is not a valid semaphore. (The glibc implementation currently does not check whether sem is valid.)


For an explanation of the terms used in this section, see attributes(7).

sem_getvalue()Thread safetyMT-Safe

Conforming to

POSIX.1-2001, POSIX.1-2008.


The value of the semaphore may already have changed by the time sem_getvalue() returns.

See Also

sem_post(3), sem_wait(3), sem_overview(7)


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Referenced By

sem_close(3), sem_open(3), sem_overview(7), sem_post(3), sem_unlink(3), sem_wait(3).

2021-03-22 Linux Programmer's Manual