implement semaphore

c_os/lib/Semaphore.cc

@@ -0,0 +1,32 @@
+#include "Semaphore.h"
+#include "kernel/Globals.h"
+
+void Semaphore::p() {
+    // Lock to allow deterministic operations on counter
+    this->lock.acquire();
+
+    if (this->counter > 0) {
+        // Semaphore can be acquired
+        this->counter = this->counter - 1;
+    } else {
+        // Block and manage thread in semaphore queue until it's woken up by v()
+        this->waitQueue.enqueue(scheduler.get_active());
+        scheduler.block();
+    }
+
+    this->lock.release();
+}
+
+void Semaphore::v() {
+    this->lock.acquire();
+
+    if (!this->waitQueue.isEmpty()) {
+        // Semaphore stays busy and unblocks next thread to work in critical section
+        scheduler.deblock((Thread*)this->waitQueue.dequeue());
+    } else {
+        // No more threads want to work so free semaphore
+        this->counter = this->counter + 1;
+    }
+
+    this->lock.release();
+}