stuff

Examples/pthreads/pthreads6.c

@@ -0,0 +1,68 @@
+/*
+ * pthreads6.c - Shows the problems that can happen due to the fact
+ *               that thread routine parameters are pointers to memory
+ *               locations.  Threads can easily affect other threads
+ *               memory.
+ *
+ *               NOTE: Context switches between threads can happen when unexpected
+ */
+
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+// Function to check if a number is prime
+//   Returns 1 if prime, 0 if not prime
+int is_prime(int v)
+{
+    for(int i = 2; i < v; i++) {
+        if(v % i == 0) {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+// Thread routine
+//    calls is_prime to check if the input argument is prime
+//    Modifies the input parameter by dereferencing the pointer
+//
+// NOTE: that parameters and return value are pointers
+void* thread_routine(void* args)
+{
+    int* val = (int*)args;
+    if(is_prime(*val)) {
+        printf("THREAD %lu FOUND that %d is prime %p\n", pthread_self(), *val, val);
+        *val = 1;
+    } else {
+        printf("THREAD %lu FOUND that %d is not prime %p\n", pthread_self(), *val, val);
+        *val = 0;
+    }
+    return NULL;
+}
+
+int main()
+{
+    int val = 0;
+
+    // Create 10 threads
+    pthread_t thr[10];
+    for(int i = 0; i < 10; i++) {
+
+        // NOTE: there is a single memory location for val which is passed to all threads
+        val = i + 2;
+        if(pthread_create(&thr[i], NULL, thread_routine, (void*)&val) == -1) {
+            printf("COULD NOT CREATE A THREAD\n");
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    // Wait for threads to finish
+    for(int i = 0; i < 10; i++) {
+        pthread_join(thr[i], NULL);
+    }
+
+    // Return success
+    return 0;
+}