C pthread, Pointer looses content

Question

I'm trying to use a returned value of a thread. For that i just found the following article: How to return a value from thread in C

So I use the following Code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *myThread()
{
   int ret = 42;
//   printf("%d\n", ret);
   printf("%p\n",(void*)&ret);

   void * ptr = (void*)&ret;
   printf("%p\n", ptr);
   printf("%d\n", *((int *)ptr));
   return (void*) &ret;
}

int main()
{
   pthread_t tid;
   static void *status;

//   int ret = 42;
//   status = &ret;
//   printf("%d\n", *((int *)status));

   pthread_create(&tid, NULL, myThread, NULL);
   pthread_join(tid, &status);

   printf("%p\n",((int *)status));  
   printf("%d\n", *((int *)status));
   return 0;
}

The outputs are: 0x7f7ead136f04, 0x7f7ead136f04, 42, 0x7f7ead136f04, 0

Why is the last value not 42?

Same Problem here:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

     void* aufgabe_drei_thread() {
         int i = 5;
         return &i; 
     }

int main(int argc, char** argv) { 
    int* ptr_wert_aus_drei;
    pthread_t thread_three_id;
    pthread_create(&thread_three_id, NULL, aufgabe_drei_thread, NULL);
    pthread_join (thread_three_id, &ptr_wert_aus_drei);
    printf("Der Wert aus Thread 3 ist: %d\n", *((int *)ptr_wert_aus_drei));

     return (EXIT_SUCCESS);
}

Output is: Der Wert aus Thread 3 ist: 32508 and not 5.

What am i doing wrong?

You can't return a pointer to a local variable. Local variables are destroyed when the function is over. — Havenard, Apr 26 '16 at 22:43

score 2 · Answer 1 · answered Apr 26 '16 at 22:49

Allocate on the heap to hang onto the value, or pass a pointer to store it.

This:

void *myThread(void *opaque)
{
   int *ret = malloc(sizeof(int));
   *ret = 42;
//   printf("%d\n", *ret);
   printf("%p\n",(void*) ret);

   void * ptr = (void*) ret;
   printf("%p\n", ptr);
   printf("%d\n", *((int *)ptr));
   return (void*) ret;
}

Or this:

void *myThread(void *opaque)
{
   int *ret = (int *) opaque;
   *ret = 42;
//   printf("%d\n", *ret);
   printf("%p\n",(void*) ret);

   void * ptr = (void*) ret;
   printf("%p\n", ptr);
   printf("%d\n", *((int *)ptr));
   return NULL;
}

While passing the variable like this:

pthread_create(&tid, NULL, myThread, &status);
pthread_join(tid, NULL);

score 2 · Answer 2 · edited Apr 26 '16 at 22:58

Declare i as global because inside aufgabe_drei_thread() i will be destroyed when the function exits.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

int i = 0;

void* aufgabe_drei_thread() { 
    i = 42;       
    return &i; 
}

int main(int argc, char** argv) { 
    int* ptr_wert_aus_drei;
    pthread_t thread_three_id;
    pthread_create(&thread_three_id, NULL, aufgabe_drei_thread, NULL);
    pthread_join (thread_three_id, &ptr_wert_aus_drei);
    printf("Der Wert aus Thread 3 ist: %d\n", *((int *)ptr_wert_aus_drei));

    return (EXIT_SUCCESS);
}

or

void* aufgabe_drei_thread() {       
    return (void*) 42; 
}

int main(int argc, char** argv) { 
    int* ptr_wert_aus_drei;
    pthread_t thread_three_id;
    pthread_create(&thread_three_id, NULL, aufgabe_drei_thread, NULL);
    pthread_join (thread_three_id, &ptr_wert_aus_drei);
    printf("Der Wert aus Thread 3 ist: %d\n", *((int *)ptr_wert_aus_drei));

    return (EXIT_SUCCESS);
}

C pthread, Pointer looses content

2 Answers2