Question

I have a question about returning results from a pthread executed function.

Related code:

void* thread_func(void* args)
{
    float result = 0;
    // ...do something
    return (void*)&result;
}

// ... using code
float answer;
pthread_join(pthread_handle, &answer);

To me, this kind of solution doesn’t seem like it should work safely because result would reside on the stack of thread_func and cease to exist after thread_func returns. However, in all the tests I’ve done it seems to work flawlessly. Is there something I’m misunderstanding about why this is safe? If not and my tests just happened to work due to some fluke, how do I safely get the return value back from thread_func safely?

Answer 1

This isn’t safe. You simply got lucky (or rather unlucky). After the function ends all automatic variables are lost (thus pointers to them are useless).

• Use malloc and return that memory
• Use a static object (a global variable for instance)
• Use memory passed by the function that will join

In conclusion use some memory that will survive after the function ends.