stuff

Examples/pthreads/pthreads5.c

@@ -0,0 +1,66 @@
+/*
+ * pthreads5.c - Shows a problem that can happen with parameters
+ *               passed to thread routines due to the fact that
+ *               the parameter must be a pointer.  The referenced
+ *               location must be valid for the entire life of the
+ *               thread.
+ *
+ *               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
+//
+// 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\n", pthread_self(), *val);
+    } else {
+        printf("THREAD %lu FOUND that %d is not prime\n", pthread_self(), *val);
+    }
+    return NULL;
+}
+
+// start_thread
+//   Creates a new thread and returns the thread ID
+//
+// NOTE: the parameter to the thread is created on the stack
+//       the address becomes invalid when the function returns
+pthread_t start_thread(int val)
+{
+    pthread_t thr;
+    printf("Before thread creation val=%d\n", val);
+    if(pthread_create(&thr, NULL, thread_routine, (void*)&val) == -1) {
+        printf("COULD NOT CREATE A THREAD\n");
+        exit(EXIT_FAILURE);
+    }
+    return thr;
+}
+
+int main()
+{
+    pthread_t thr1 = start_thread(2);
+    pthread_t thr2 = start_thread(100);
+    pthread_join(thr1, NULL);
+    pthread_join(thr2, NULL);
+    return 0;
+}