sortix--sortix/libc/semaphore/sem_trywait.c

/*
 * Copyright (c) 2014 Jonas 'Sortie' Termansen.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 * semaphore/sem_trywait.c
 * Lock a semaphore.
 */

#include <errno.h>
#include <semaphore.h>
#include <stdbool.h>

int sem_trywait(sem_t* sem)
{
	int old_value = __atomic_load_n(&sem->value, __ATOMIC_SEQ_CST);
	if ( old_value <= 0 )
			return errno = EAGAIN, -1;
	int new_value = old_value - 1;
	if ( !__atomic_compare_exchange_n(&sem->value, &old_value, new_value, false,
	                                  __ATOMIC_SEQ_CST, __ATOMIC_RELAXED) )
			return errno = EAGAIN, -1;
	return 0;
}