From fbe2c1f2ceb855a8bfb78359c55a6f763ed5210b Mon Sep 17 00:00:00 2001
From: Kevoot <kevinholmes88@gmail.com>
Date: Sat, 30 Jun 2018 18:45:41 -0400
Subject: [PATCH] Add Semaphore (#125)

---
 nx/include/switch.h                  |  1 +
 nx/include/switch/kernel/semaphore.h | 44 ++++++++++++++++++++++++++++
 nx/source/kernel/semaphore.c         | 37 +++++++++++++++++++++++
 3 files changed, 82 insertions(+)
 create mode 100644 nx/include/switch/kernel/semaphore.h
 create mode 100644 nx/source/kernel/semaphore.c

diff --git a/nx/include/switch.h b/nx/include/switch.h
index 2ed024d1..c23aeea2 100644
--- a/nx/include/switch.h
+++ b/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"
diff --git a/nx/include/switch/kernel/semaphore.h b/nx/include/switch/kernel/semaphore.h
new file mode 100644
index 00000000..4eaa434b
--- /dev/null
+++ b/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);
diff --git a/nx/source/kernel/semaphore.c b/nx/source/kernel/semaphore.c
new file mode 100644
index 00000000..328e8414
--- /dev/null
+++ b/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;
+}