How to return a value from pthread threads in C?

Question

You are returning the address of a local variable, which no longer exists when the thread function exits. In any case, why call pthread_exit? why not simply return a value from the thread function?

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

and then in main:

printf("%d\n", (int)status);   

If you need to return a complicated value such a structure, it's probably easiest to allocate it dynamically via malloc() and return a pointer. Of course, the code that initiated the thread will then be responsible for freeing the memory.

Here is a correct solution. In this case tdata is allocated in the main thread, and there is a space for the thread to place its result.

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

typedef struct thread_data {
   int a;
   int b;
   int result;

} thread_data;

void *myThread(void *arg)
{
   thread_data *tdata=(thread_data *)arg;

   int a=tdata->a;
   int b=tdata->b;
   int result=a+b;

   tdata->result=result;
   pthread_exit(NULL);
}

int main()
{
   pthread_t tid;
   thread_data tdata;

   tdata.a=10;
   tdata.b=32;

   pthread_create(&tid, NULL, myThread, (void *)&tdata);
   pthread_join(tid, NULL);

   printf("%d + %d = %d\n", tdata.a, tdata.b, tdata.result);   

   return 0;
}

You've returned a pointer to a local variable. That's bad even if threads aren't involved.

The usual way to do this, when the thread that starts is the same thread that joins, would be to pass a pointer to an int, in a location managed by the caller, as the 4th parameter of pthread_create. This then becomes the (only) parameter to the thread's entry-point. You can (if you like) use the thread exit value to indicate success:

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

int something_worked(void) {
    /* thread operation might fail, so here's a silly example */
    void *p = malloc(10);
    free(p);
    return p ? 1 : 0;
}

void *myThread(void *result)
{
   if (something_worked()) {
       *((int*)result) = 42;
       pthread_exit(result);
   } else {
       pthread_exit(0);
   }
}

int main()
{
   pthread_t tid;
   void *status = 0;
   int result;

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

   if (status != 0) {
       printf("%d\n",result);
   } else {
       printf("thread failed\n");
   }

   return 0;
}

If you absolutely have to use the thread exit value for a structure, then you'll have to dynamically allocate it (and make sure that whoever joins the thread frees it). That's not ideal, though.

I think you have to store the number on heap. The int ret variable was on stack and was destructed at the end of execution of function myThread.

void *myThread()
{
   int *ret = malloc(sizeof(int));
   if (ret == NULL) {
       // ...
   }
   *ret = 42;
   pthread_exit(ret);
}

Don't forget to free it when you don't need it

Another solution is to return the number as value of the pointer, like Neil Butterworth suggests.