Question

When creating a new pthread and passing it an argument modified with pthread_attr_getstack it seems to not using the defined stack space.

void* thread_function(void * ptr)
{
  int a;
  printf("stack var in thread %p\n",&a);
}


int main( int argc , char **argv )
{
  pthread_t thread;
  void * ptr = NULL;
  const int stack_size = 10*1024;

  void * stack = malloc(stack_size);
  printf("alloc=%p\n",&stack);

  pthread_attr_t attr;
  pthread_attr_init(&attr);
  pthread_attr_setstack( &attr , stack , stack_size );

  if (pthread_create(&thread, &attr, &thread_function , ptr ) ) {
    printf("failed to create thread\n");
    return 1;
  }

  pthread_attr_destroy(&attr);
  pthread_exit( 0 );
  return 0;
}

Unfortunately the output is:

alloc=0x7fff48989bc8
stack var in thread 0x7f6e6f0d2ebc

Even if stack grows backward (which i am not sure) the pointer values differ so much, that only hope that the created thread uses a different virtual memory address space. But i think this is not the case.

Answer 1

You’re printing the wrong thing,

printf("alloc=%p\n",&stack);

Prints the address of a local stack variable, not the allocated memory. You have to do

printf("alloc=%p\n",stack);

Also, you need to check for errors:

ERRORS

   pthread_attr_setstack() can fail with the following error:

   EINVAL stacksize is less than PTHREAD_STACK_MIN (16384) bytes.  On some
          systems, this error may also occur if stackaddr or
          stackaddr + stacksize is not suitably aligned.

You’ve only set a stack of 10kB, so try again with a bigger stack, and check the return value of pthread_attr_setstack.

I’d probably try to make the stack page aligned as well.