[SOLVED] Very simple pthread program sometimes prints garbage value for a variable

gregorian · 05-16-2012, 08:32 AM

Code:

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

void* multi(void* tid)
{
        int id = *((int*)tid);
        printf("%d\n",id);
}

int main()
{
        int a = 1;
        pthread_t thread1;
        pthread_create(&thread1, NULL, multi, (void*)(&a));
        pthread_exit(NULL);
        return 0;
}

If you run this program using watch -n 0.1 ./a.out you will occasionally see garbage values being printed. According to the pthread_exit man page, the main function will be alive till the child thread completes. If that's the case, 'a' should still be present on the stack, and I should never get garbage values.

NevemTeve · 05-16-2012, 08:47 AM

I suggest you remove pthread_exit from main, and see if the problem still persists.

dwhitney67 · 05-16-2012, 09:13 AM

Quote:

Originally Posted by gregorian

According to the pthread_exit man page, the main function will be alive till the child thread completes. If that's the case, 'a' should still be present on the stack, and I should never get garbage values.

According to the man page, when pthread_exit() is called from the main function, the variable used for the return value (retval) should not be declared on the stack within main, for it will be destroyed when the main thread exits. One has to assume that this same principle applies to other variables within the main function that are also declared on the stack.

If you declare 'a' outside of main, say as a global variable, then your program should function as expected.

gregorian · 05-16-2012, 10:39 AM

Quote:

Originally Posted by NevemTeve

I suggest you remove pthread_exit from main, and see if the problem still persists.

If I remove pthread_exit from main, the process will terminate even if the threads do not. I don't want this to happen.

Quote:

According to the man page, when pthread_exit() is called from the main function, the variable used for the return value (retval) should not be declared on the stack within main, for it will be destroyed when the main thread exits. One has to assume that this same principle applies to other variables within the main function that are also declared on the stack.

But the main thread does not exit when you call pthread_exit from within it according to the man page. So my variable should be alive. Please refer to the 'Notes' section:
http://linux.die.net/man/3/pthread_exit

I have also tested this case by making the thread perform infinite calculation. The process does not terminate.

Quote:

If you declare 'a' outside of main, say as a global variable, then your program should function as expected.

Yes, I noticed this, but I don't want to be forced to use global variables. It will become unwieldly in large programs.

dwhitney67 · 05-16-2012, 10:49 AM

Quote:

Originally Posted by gregorian

But the main thread does not exit when you call pthread_exit from within it according to the man page. So my variable should be alive.[/url]

The main thread does exit! And as for the local variables declared on the stack, they are trashed. Try this version of your program; notice that the printf() in main is never executed.

Code:

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

void* thread(void* arg)
{
    int id = *((int*) arg);

    printf("%d\n", id);

    return NULL;
}

int main()
{
    pthread_t tid;
    int       id = 1;

    pthread_create(&tid, NULL, thread, &id);

    pthread_exit(NULL);

    printf("The main thread should never reach this point.\n");

    return 0;
}

In examining the bigger picture, why do you want the main function to exit? You could leave it running by having it block on an individual thread using pthread_join().

gregorian · 05-16-2012, 10:57 AM

Oh I get your point now. Thanks a lot!

NevemTeve · 05-16-2012, 11:22 AM

> > I suggest you remove pthread_exit from main, and see if the problem still persists.

> If I remove pthread_exit from main, the process will terminate even if the threads do not. I don't want this to happen.

Then pthread_join is your friend, not pthread_exit.

pan64 · 05-16-2012, 11:30 AM

and also there are some OSs where exiting the main thread will destroy all of the threads.