Wrong return value after terminating a thread

Question

I am approaching to pthreads in C and I'm starting writing very dumb programs to get the hang of them. I tried to create a program containing two threads, they should print their name and their status should be collected as they terminate their execution. So my code is:

//Function declaration
void *state_your_name(char*);

//Function definition

//Passing a string as parameter for 
//outputting the name of the thread
void *state_your_name(char *name) {
    //Declaration of the variable containing the return status
    void* status;

    //Printing out the string
    printf("%s\n", name);
    //Exiting the thread and saving the return value
    pthread_exit(status);
}

int main(void) {
    pthread_t tid_1, tid_2;
    void * status;

    //Creating thread 1...
    if (pthread_create(&tid_1, NULL, state_your_name, "Thread 1")) {
        printf("Error creating thread 1");
        exit(1);
    }

    //Creating thread 2...    
    if (pthread_create(&tid_2, NULL, state_your_name, "Thread 2")) {
        printf("Error creating thread 2");
        exit(1);
    }

    //Waiting for thread 1 to terminate and 
    //collecting the return value...    
    if (pthread_join(tid_1, (void *) &status)) {
        printf("Error joining thread 1");
        exit(1);
    }
        printf("Thread 1 - Return value: %d\n", (int)status );

    //Waiting for thread 2 to terminate and 
    //collecting the return value...      
    if (pthread_join(tid_2, (void *) &status)) {
        printf("Error joining thread 2");
        exit(1);
    }
        printf("Thread 2 - Return value: %d\n", (int)status );

    return 0;
}

I would expect an output like this:

Thread 1
Thread 2
Thread 1 - Return value: 0
Thread 2 - Return value: 0

but my issue is the return value of Thread 1 is 733029576, but Thread 2 returns 0 as predicted; it's like the status variable is uninitialized and containing garbage. What am I missing?

D'oh, you never initialize `status`. So it's just undefined behaviour. — Kerrek SB, Jun 10 '13 at 16:39
how do you determine your return value of thread1 has that value? — Nicholaz, Jun 10 '13 at 16:42

score 5 · Accepted Answer · answered Jun 10 '13 at 16:39

The reason you see a garbage value in the output is that the local void *status variable of state_your_name is uninitialized:

void *state_your_name(char *name) {
    //Declaration of the variable containing the return status
    void* status = NULL; // <<=====    Add initialization here

    //Printing out the string
    printf("%s\n", name);
    //Exiting the thread and saving the return value
    pthread_exit(status);
}

With this change in place, your program should produce the output that you expect.

Note that returning status directly from state_your_name is an alternative to calling pthread_exit: you can replace that call with return status.

score 2 · Answer 2 · answered Jun 10 '13 at 16:39

2

Your code in state_your_name() does not initialize the status that it returns, so you get indeterminate information back. You're unlucky that you're getting zero rather than garbage.

answered Jun 10 '13 at 16:39

Jonathan Leffler

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score 2 · Answer 3 · edited May 23 '17 at 12:00

2

You are returning a pointer to status that is defined in the thread's stack. That memory may go away after the thread exits. You should not declare status inside your thread. You are better off just doing return 0 inside state_your_name().

See How to return a value from thread in C

edited May 23 '17 at 12:00

Community

1
1

answered Jun 10 '13 at 16:49

Markku K.

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score 0 · Answer 4 · answered Jun 10 '13 at 16:44

To get a predictably different return value, try this

void *state_your_name(char *name) {
    //Declaration of the variable containing the return status
    void* status= (void *)(int)name[0];
    ....

This will return the ascii code of the first letter of the name.

Also, the implementations of pthread_join I know, take a void** as last parameter, so

if (pthread_join(tid_2, &status)) {

would be correct (your casting it (void*) being sort of misleading)

Wrong return value after terminating a thread

4 Answers4