SEM_GETVALUE

Section: Linux Programmer's Manual (3)
Updated: 2020-06-09
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NAME

sem_getvalue - get the value of a semaphore

SYNOPSIS

#include <semaphore.h>

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

Link with -pthread.

DESCRIPTION

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.

ERRORS

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

ATTRIBUTES

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

Interface	Attribute	Value
sem_getvalue()	Thread safety	MT-Safe

CONFORMING TO

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

NOTES

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

COLOPHON

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