Question

The following program shows that we can use return or pthread_exit to return a void* variable that is available to pthread_join‘s status variable.

1. Should there be a preference for using one over the other?

2. Why does using return work? Normally we think of return putting a value on the stack but since the thread is completed the stack should vanish. Or does the stack not get destroyed until after pthread_join?

3. In your work, do you see much use of the status variable? It seems 90% of the code I see just NULLs out the status parameter. Since anything changed via the void* ptr is already reflected in the calling thread there doesn’t seem much point to returning it. Any new void* ptr returned would have to point to something malloced by the start thread, which leaves the receiving thread with the responsibility to dispose of it. Am I wrong in thinking the status variable is semi-pointless?

Here is the code:

#include <iostream>
#include <pthread.h>

using namespace std;

struct taskdata
{
       int  x;
     float  y;
    string  z;
};


void* task1(void *data)
{
    taskdata *t = (taskdata *) data;

    t->x += 25;
    t->y -= 4.5;
    t->z = "Goodbye";

    return(data);
}

void* task2(void *data)
{
    taskdata *t = (taskdata *) data;

    t->x -= 25;
    t->y += 4.5;
    t->z = "World";

    pthread_exit(data);
}


int main(int argc, char *argv[])
{
    pthread_t threadID;

    taskdata t = {10, 10.0, "Hello"};

    void *status;

    cout << "before " << t.x << " " << t.y << " " << t.z << endl;

    //by return()

    pthread_create(&threadID, NULL, task1, (void *) &t);

    pthread_join(threadID, &status);

    taskdata *ts = (taskdata *) status;

    cout << "after task1 " << ts->x << " " << ts->y << " " << ts->z << endl;

    //by pthread_exit()

    pthread_create(&threadID, NULL, task2, (void *) &t);

    pthread_join(threadID, &status);

    ts = (taskdata *) status;

    cout << "after task2 " << ts->x << " " << ts->y << " " << ts->z << endl;

}

With output of:

before 10 10 Hello
after task1 35 5.5 Goodbye
after task2 10 10 World

Answer 1

(1) In C++ code, using return causes the stack to be unwound and local variables destroyed, whereas pthread_exit is only guaranteed to invoke cancellation handlers registered with pthread_cancel_push(). On some systems this mechanism will also cause the destructors for C++ local variables to be called, but this is not guaranteed for portable code — check your platform documentation.

Also, in main(), return will implicitly call exit(), and thus terminate the program, whereas pthread_exit() will merely terminate the thread, and the program will remain running until all threads have terminated or some thread calls exit(), abort() or another function that terminates the program.

(2) The use of return works because the POSIX specification says so. The returned value is stored in a place where pthread_join() can retrieve it. The resources used by the thread are not reclaimed until pthread_join() is called.

(3) I never use the return value of a thread in raw POSIX threads. However, I tend to use higher level facilities such as the Boost thread library, and more recently the C++0x thread library, which provide alternative means for transferring values between threads such as futures, which avoid the problems associated with memory management that you allude to.