Man

Command Section
SEM_WAIT(3)            FreeBSD Library Functions Manual            SEM_WAIT(3)

NAME
     sem_wait, sem_trywait - decrement (lock) a semaphore

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <semaphore.h>

     int
     sem_wait(sem_t *sem);

     int
     sem_trywait(sem_t *sem);

DESCRIPTION
     The sem_wait() function decrements (locks) the semaphore pointed to by
     sem, but blocks if the value of sem is zero, until the value is non-zero
     and the value can be decremented.

     The sem_trywait() function decrements (locks) the semaphore pointed to by
     sem only if the value is non-zero.  Otherwise, the semaphore is not
     decremented and an error is returned.

RETURN VALUES
     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The sem_wait() and sem_trywait() functions will fail if:

     [EINVAL]           The sem argument points to an invalid semaphore.

     Additionally, sem_wait() will fail if:

     [EINTR]            A signal interrupted this function.

     Additionally, sem_trywait() will fail if:

     [EAGAIN]           The semaphore value was zero, and thus could not be
                        decremented.

SEE ALSO
     sem_getvalue(3), sem_post(3), sem_timedwait(3)

STANDARDS
     The sem_wait() and sem_trywait() functions conform to ISO/IEC 9945-1:1996
     (``POSIX.1'').

FreeBSD 11.1-RELEASE-p4         April 16, 2013         FreeBSD 11.1-RELEASE-p4
Command Section