pthread return value error

I am new to Linux programming.

I am returning a value from thread. But when compiled it is listing some errors. I am listing the code and error below. Please help me to understand why the error and how to solve it.

code

#include <pthread.h>
#include <stdio.h>
void* compute_prime (void* arg)
{
        int x = 2;
        return (void*) x;
}


int main ()
{
        pthread_t thread;
        int prime;
        pthread_create (&thread, 开发者_开发百科NULL, &compute_prime, NULL);
        pthread_join (thread, (void*) &prime);
        printf("The returned value is %d.\n", prime);
        return 0;
}

error

$ g++ -othj pdfex.cpp -lpthread
pdfex.cpp: In function `int main()':
pdfex.cpp:17: error: invalid conversion from `void*' to `void**'
pdfex.cpp:17: error:   initializing argument 2 of `int pthread_join(pthread_t, void**)'

What am I doing wrong?

---

Since the declaration of pthread_join() is:

int pthread_join(pthread_t thread, void **value_ptr);

your '(void *)' cast is wrong - and the compiler is telling you that.

How to fix?

• If sizeof(void *) == sizeof(int) on your machine, then:

  pthread_join(thread, (void **)&prime);
  

• Otherwise:

  uintptr_t uip;
  pthread_join(thread, (void **)&uip);
  prime = uip;
  

  That requires #include <stdint.h> (or #include <inttypes.h>), and exploits the fact that a uintptr_t is the same size as a void *.

---

This code delivers the answer 2 when compiled for 64-bit on MacOS X 10.6.4 (which corresponds to the 'otherwise' clause):

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

static void *compute_prime(void* arg)
{
    uintptr_t x = 2;
    assert(arg == 0);
    return (void *)x;
}

int main(void)
{
    pthread_t thread;
    uintptr_t prime;
    pthread_create(&thread, NULL, &compute_prime, NULL);
    pthread_join(thread, (void **) &prime);
    printf("The returned value is %" PRIuPTR ".\n", prime);
    return 0;
}

---

pthread_join() is asking for a void**, and you are giving it a void*.