implement semaphore

c_os/kernel/threads/Scheduler.cc

@@ -181,7 +181,6 @@ void Scheduler::block() {
     }
 
     cpu.disable_int();
-
     Thread& next = *(Thread*)this->readyQueue.dequeue();
     this->dispatch(next);
 }

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();
+}

c_os/lib/SpinLock.cc

@@ -50,6 +50,8 @@ static inline unsigned long CAS(unsigned long* ptr, unsigned long old, unsigned
  * Beschreibung:    Lock belegen.                                            *
  *****************************************************************************/
 void SpinLock::acquire() {
+    // If lock == 0 the SpinLock can be aquired without waiting
+    // If lock == 1 the  while loop blocks until aquired
     while (CAS(ptr, 0, 1) != 0) {}
 }
 

c_os/main.cc

@@ -84,6 +84,11 @@ int main() {
     scheduler.schedule();
 
     // TODO: Use templates for queue so threads don't have to be casted down from chain
+    // TODO: Change scheduler to only use references instead of pointers
+    // TODO: Rewrite all demos to threads
+    // TODO: Make menu for demos
+    // TODO: Unify debug output format
+    // TODO: Serial output
 
     // Scheduler doesn't return
     return 0;