Lab5: add pthread example 1

lab5/demos/pthread_count_1.c

@@ -0,0 +1,52 @@
+/*
+ * This program ends up in a threadrace when the argument is around 1M
+ *
+ * We need synchronization primitives to help here!
+ */
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+struct args {
+    int max;
+    char letter;
+};
+
+volatile int counter = 0;
+
+void* count_thread(void* arg) {
+    struct args* a = arg;
+
+    for (int i = 0; i < a->max; i++) {
+        counter = counter + 1;
+    }
+
+    printf("Thread %c: done with counter at %d\n", a->letter, counter);
+    return NULL;
+}
+
+void* make_args(int max, char letter) {
+    struct args* a = malloc(sizeof(struct args));
+    a->max = max;
+    a->letter = letter;
+
+    return (void*) a;
+}
+
+int main(int argc, char* argv[]) {
+    if (argc != 2) {
+        exit(1);
+    }
+
+    int max = atoi(argv[1]);
+
+    pthread_t p1, p2;
+    pthread_create(&p1, NULL, count_thread, make_args(max, 'A'));
+    pthread_create(&p2, NULL, count_thread, make_args(max, 'B'));
+
+    pthread_join(p1, NULL);
+    pthread_join(p2, NULL);
+
+    printf("main: done\n [counter: %d]\n [should: %d]\n", counter, max * 2);
+    return 0;
+}