From 200f22578700b82c5a2d2052c9fda0d9e431d25f Mon Sep 17 00:00:00 2001
From: Stefan Zabka <zabkaste@hu-berlin.de>
Date: Fri, 12 Jun 2020 00:05:28 +0200
Subject: THIS SHIT WORKS

---
 04_exercise/rwlock/rwlock.c | 67 +++++++++++++++++++++++++++++++++++++++++++++
 04_exercise/rwlock/rwlock.h | 16 +++++++++++
 2 files changed, 83 insertions(+)
 create mode 100644 04_exercise/rwlock/rwlock.c
 create mode 100644 04_exercise/rwlock/rwlock.h

(limited to '04_exercise/rwlock')

diff --git a/04_exercise/rwlock/rwlock.c b/04_exercise/rwlock/rwlock.c
new file mode 100644
index 0000000..3de5083
--- /dev/null
+++ b/04_exercise/rwlock/rwlock.c
@@ -0,0 +1,67 @@
+//
+// Created by stefan on 11.06.20.
+//
+
+#include "rwlock.h"
+#include <stdbool.h>
+#include <stdio.h>
+#include <pthread.h>
+#include <stdlib.h>
+
+static const bool DEBUG = false;
+
+
+void rwLockWrite(atomic_char *lock) {
+    char status;
+    bool succeeded;
+    do {
+        status = RW_UNLOCKED;
+        succeeded = atomic_compare_exchange_weak(lock, &status, RW_WRITE_LOCKED);
+    } while (!succeeded);
+    if(DEBUG) {
+        fprintf(stderr, "Thread %lu: Aquired Write Lock \n", pthread_self() % 1000);
+    }
+
+}
+
+void rwUnlockWrite(atomic_char *lock) {
+    char expected = RW_WRITE_LOCKED;
+    if (!atomic_compare_exchange_strong(lock, &expected, RW_UNLOCKED)) {
+        fprintf(stderr, "Couldn't unlock from writelock. Lock was at %d", expected);
+        perror("Lock got into undefined state");
+        exit(-1);
+    }
+    if(DEBUG) {
+        fprintf(stderr, "Thread %lu: Released Write Lock \n", pthread_self() % 1000);
+    }
+    if (sched_yield() < 0) {
+        perror("Couldn't sleep. This shouldn't happen.");
+        exit(-1);
+    }
+
+}
+void rwLockRead(atomic_char *lock) {
+    while (atomic_load(lock) == RW_WRITE_LOCKED) {
+
+    }
+    atomic_fetch_add(lock, 1);
+    if(DEBUG) {
+        fprintf(stderr, "Thread %lu: Aquired Read Lock \n", pthread_self() % 1000);
+    }
+}
+void rwUnlockRead(atomic_char *lock) {
+    char value = atomic_load(lock);
+    if (value < 1) {
+        fprintf(stderr, "Couldn't unlock from readlock. Lock was at %d", value);
+        perror("Lock got into undefined state");
+        exit(-1);
+    }
+    atomic_fetch_sub(lock, 1);
+    if(DEBUG) {
+        fprintf(stderr, "Thread %lu: Released Read Lock \n", pthread_self() % 1000);
+    }
+    if (sched_yield() < 0) {
+        perror("Couldn't sleep. This shouldn't happen.");
+        exit(-1);
+    }
+}
\ No newline at end of file
diff --git a/04_exercise/rwlock/rwlock.h b/04_exercise/rwlock/rwlock.h
new file mode 100644
index 0000000..aa27f09
--- /dev/null
+++ b/04_exercise/rwlock/rwlock.h
@@ -0,0 +1,16 @@
+//
+// Created by stefan on 11.06.20.
+//
+
+#ifndef BETRIEBSYSTEME_RWLOCK_H
+#define BETRIEBSYSTEME_RWLOCK_H
+#include <stdatomic.h>
+
+static const char RW_WRITE_LOCKED = -1;
+static const char RW_UNLOCKED = 0;
+void rwLockWrite(atomic_char *lock);
+void rwUnlockWrite(atomic_char *lock);
+void rwLockRead(atomic_char *lock);
+void rwUnlockRead(atomic_char *lock);
+
+#endif // BETRIEBSYSTEME_RWLOCK_H
-- 
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