freebsd man page for sem_trywait

SEM_WAIT(3)						   BSD 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'').

BSD
								  April 16, 2013							       BSD