SEM_GETVALUE(3) | Library Functions Manual | SEM_GETVALUE(3)

# NAME

`sem_getvalue` — get the value of a semaphore

# SYNOPSIS

POSIX Threads Library (libpthread, -lpthread)
`#include <semaphore.h>`

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

# DESCRIPTION

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.

# RETURN VALUES

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.

# ERRORS

`sem_getvalue`() will fail if:

[`EINVAL`]
  sem points to an invalid semaphore.

# SEE ALSO

sem_destroy(3), sem_init(3), sem_open(3), sem_post(3), sem_wait(3)

# STANDARDS

`sem_getvalue`() 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.

June 7, 2025 | Debian