Add Semaphore (#125)

nx/include/switch.h

@@ -25,6 +25,7 @@ extern "C" {
 #include "switch/kernel/rwlock.h"
 #include "switch/kernel/condvar.h"
 #include "switch/kernel/thread.h"
+#include "switch/kernel/semaphore.h"
 #include "switch/kernel/virtmem.h"
 #include "switch/kernel/detect.h"
 #include "switch/kernel/random.h"

nx/include/switch/kernel/semaphore.h

@@ -0,0 +1,44 @@
+/**
+ * @file semaphore.h
+ * @brief Thread synchronization based on Mutex.
+ * @author SciresM & Kevoot
+ * @copyright libnx Authors
+ */
+#pragma once
+
+#include "mutex.h"
+#include "condvar.h"
+
+/// Semaphore structure.
+typedef struct Semaphore
+{
+    CondVar condvar; ///< Conditional Variable Object.
+    Mutex   mutex;   ///< Mutex Object.
+    u64     count;   ///< Internal Counter.
+} Semaphore;
+
+/**
+ * @brief Initializes a semaphore and its internal counter.
+ * @param s Semaphore object.
+ * @param initial_count initial value for internal counter (typically the # of free resources).
+ */
+void semaphoreInit(Semaphore *s, u64 initial_count);
+
+/**
+ * @brief Increments the Semaphore to allow other threads to continue.
+ * @param s Semaphore object.
+ */
+void semaphoreSignal(Semaphore *s);
+
+/**
+ * @brief Decrements Semaphore and waits if 0.
+ * @param s Semaphore object.
+ */
+void semaphoreWait(Semaphore *s);
+
+/**
+ * @brief Attempts to get lock without waiting.
+ * @param s Semaphore object.
+ * @return true if no wait and successful lock, false otherwise.
+ */
+bool semaphoreTryWait(Semaphore *s);

nx/source/kernel/semaphore.c

@@ -0,0 +1,37 @@
+// Copyright 2018 Kevoot
+#include "kernel/semaphore.h"
+#include "kernel/svc.h"
+
+void semaphoreInit(Semaphore *s, u64 initial_count) {
+    s->count = initial_count;
+    mutexInit(&s->mutex);
+    condvarInit(&s->condvar, &s->mutex);
+}
+
+void semaphoreSignal(Semaphore *s) {
+    mutexLock(&s->mutex);
+    s->count++;
+    condvarWakeOne(&s->condvar);
+    mutexUnlock(&s->mutex);
+}
+
+void semaphoreWait(Semaphore *s) {
+    mutexLock(&s->mutex);
+    // Wait until signalled.
+    while (!s->count) {
+        condvarWait(&s->condvar);
+    }
+    s->count--;
+    mutexUnlock(&s->mutex);
+}
+
+bool semaphoreTryWait(Semaphore *s) {
+    mutexLock(&s->mutex);
+    bool success = false;
+    // Check and immediately return success.
+    if (s->count) {
+        s->count--;
+        success = true;
+    }
+    return success;
+}