LibC: Implement unnamed semaphores

userspace/libraries/LibC/include/semaphore.h

@@ -10,7 +10,11 @@ __BEGIN_DECLS
 #include <fcntl.h>
 #include <time.h>
 
-typedef void* sem_t;
+typedef struct
+{
+	int shared;
+	uint32_t value;
+} sem_t;
 
 #define SEM_FAILED ((sem_t*)0)
 

userspace/libraries/LibC/semaphore.cpp

@@ -1,47 +1,52 @@
-#include <BAN/Debug.h>
+#include <BAN/Atomic.h>
 
 #include <errno.h>
 #include <semaphore.h>
+#include <sys/futex.h>
 
 int sem_destroy(sem_t* sem)
 {
 	(void)sem;
-	dwarnln("TODO: sem_destroy");
+	return 0;
-	errno = ENOTSUP;
-	return -1;
 }
 
 int sem_init(sem_t* sem, int pshared, unsigned value)
 {
-	(void)sem;
+	*sem = {
-	(void)pshared;
+		.shared = pshared,
-	(void)value;
+		.value = value,
-	dwarnln("TODO: sem_init");
+	};
-	errno = ENOTSUP;
+	return 0;
-	return -1;
 }
 
 int sem_post(sem_t* sem)
 {
-	(void)sem;
+	const auto old = BAN::atomic_fetch_add(sem->value, 1);
-	dwarnln("TODO: sem_post");
+	if (old == 0)
-	errno = ENOTSUP;
+		futex(FUTEX_WAKE, &sem->value, 1, nullptr);
-	return -1;
+	return 0;
-
 }
 
 int sem_trywait(sem_t* sem)
 {
-	(void)sem;
+	uint32_t expected = BAN::atomic_load(sem->value);
-	dwarnln("TODO: sem_trywait");
+	while (expected)
-	errno = ENOTSUP;
+		if (BAN::atomic_compare_exchange(sem->value, expected, expected - 1))
+			return 0;
+	errno = EAGAIN;
 	return -1;
 }
 
 int sem_wait(sem_t* sem)
 {
-	(void)sem;
+	for (;;)
-	dwarnln("TODO: sem_wait");
+	{
-	errno = ENOTSUP;
+		uint32_t expected = BAN::atomic_load(sem->value);
+		if (expected > 0 && BAN::atomic_compare_exchange(sem->value, expected, expected - 1))
+			return 0;
+
+		const int op = FUTEX_WAIT | (sem->shared ? 0 : FUTEX_PRIVATE);
+		if (futex(op, &sem->value, expected, nullptr) == -1 && errno == EINTR)
 			return -1;
+	}
 }