Übung2

Merged cars.c from my branch to Übung 2 branch

Ubung02.c

+// Aufgaben 1 und 2
+
+#include <pthread.h>
+#include <stdlib.h> 
+#include <stdio.h>
+#include <unistd.h>
+
+#define NUM_THREADS 2
+#define NUM_CROSSINGS 100000
+
+int cars[2];       //shared data
+int cars_on_bridge = 0;
+pthread_mutex_t mutex; 
+
+void lock(pthread_mutex_t *mutex)
+{
+    if(pthread_mutex_lock(mutex) != 0)
+    {
+        perror("lock");
+        exit(1);
+    }
+}
+
+void unlock(pthread_mutex_t *mutex)
+{
+    if(pthread_mutex_unlock(mutex) != 0)
+    ;
+    {
+        perror("unlock");
+        exit(1);
+    }
+}
+
+void *bridge_cars (void *threadid)    
+{
+    int car_number = *(int*) threadid;
+    int num_crossings = 0;
+    while(num_crossings < NUM_CROSSINGS)
+    {
+        lock(&mutex);
+        // begin of the critical section 
+        //The block inside the if statement checks if there are no cars on the bridge, as this is where the shared variable cars_on_bridge is being accessed and modified by multiple threads. 
+        if(cars_on_bridge == 0)
+        {
+            cars_on_bridge ++;
+            printf("Car %d is on the bridge.\n", car_number);
+            unlock(&mutex);
+            int crossing_time = rand() % 60 + 1;
+            usleep(crossing_time * 1000);
+            cars_on_bridge --;
+            lock(&mutex);
+            printf("Car %d leaves the bride.\n", car_number);
+            int cars_on_bridge = 0;
+            num_crossings++;
+        } 
+        //end of the critical section
+        unlock(&mutex);
+        usleep(100);
+     }
+     return NULL;
+}
+
+int main ()
+{
+    pthread_t threads[NUM_THREADS];
+    int threadid[NUM_THREADS];
+    int rc;
+    int i;
+
+
+    //init data
+    srand ((unsigned) time(NULL));
+    int car1 = 1;
+    int car2 = 2;
+
+    //initialize mutex
+    if (pthread_mutex_init(&mutex, NULL) != 0)
+    {
+        perror("pthread_mutex_init");
+        exit(1);
+    }
+
+
+    //creating threads
+    for(i = 0; i < NUM_THREADS; i++)
+    {
+        printf("In main: creating thread %d\n", i);
+        threadid[i] = i;
+        rc = pthread_create (&threads[i], NULL, bridge_cars, (void*)&threadid[i]);
+        if (rc)
+        {
+            printf("ERROR, return code from pthread_create() is %d\n", rc); 
+            exit(1);
+        }
+    }
+
+    for (i = 0; i < NUM_THREADS; i++)
+    {
+        pthread_join(threads[i], NULL);
+    }
+
+    for(i = 0; i < 100000; i++ )
+    {
+        usleep(100);
+    }
+    pthread_mutex_destroy(&mutex);
+    pthread_exit (NULL);
+}
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