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Man Pages
SEM_GETVALUE(3) FreeBSD Library Functions Manual SEM_GETVALUE(3)

sem_getvalue
get the value of a semaphore

Standard C Library (libc, -lc)

#include <semaphore.h>
int
sem_getvalue(sem_t * restrict sem, int * restrict sval);

The sem_getvalue() function sets the variable pointed to by sval to the current value of the semaphore pointed to by sem, as of the time that the call to sem_getvalue() is actually run.

The sem_getvalue() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The sem_getvalue() function will fail if:
[]
The sem argument points to an invalid semaphore.

sem_post(3), sem_trywait(3), sem_wait(3)

The sem_getvalue() function conforms to ISO/IEC 9945-1:1996 (“POSIX.1”).
The value of the semaphore is never negative, even if there are threads blocked on the semaphore. POSIX is somewhat ambiguous in its wording with regard to what the value of the semaphore should be if there are blocked waiting threads, but this behavior is conformant, given the wording of the specification.
February 15, 2000 FreeBSD 12.0-RELEASE

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