implement POSIX semaphores

src/thread/sem_timedwait.c

@@ -0,0 +1,18 @@
+#include <semaphore.h>
+#include "pthread_impl.h"
+
+int sem_timedwait(sem_t *sem, const struct timespec *at)
+{
+	int val;
+
+	for (;;) {
+		if (a_fetch_add(sem->__val, -1) > 0) return 0;
+		val = a_fetch_add(sem->__val, 1);
+		CANCELPT_BEGIN;
+		if (val <= 0 && __timedwait(sem->__val, val, CLOCK_REALTIME, at, 0) == ETIMEDOUT) {
+			errno = ETIMEDOUT;
+			return -1;
+		}
+		CANCELPT_END;
+	}
+}