thrs

mcy · 05-07-2012, 05:14 AM

hello everyone, i am making a program using threads and i've got the following function and it prints for the same thread the same tid but in different iteration.. i can't understand how it works.. there shouldn't be different tids for the father for each iteration? Please someone tell me what those tids are for each iteration.. thank u!

void *function(void *num)
{
pthread_t tid;
int i = (int) num;
while(i>0) {
i--;
pthread_create(&tid, NULL, function, (void *) i);
printf("%d %d %d\n", i, pthread_self(),tid);
pthread_join(tid, NULL); }
pthread_exit(NULL); }

dwhitney67 · 05-07-2012, 05:52 AM

When a thread is invoked using pthread_create(), a thread ID, or TID, is returned. When calling pthread_self() from within the thread, the value returned should be the same as the TID obtained by the parent.

Running your code with a small value of 'i' (in my test, set to 2), I got the following results:

Code:

  140164009686784 140164009678592     <--- From the parent thread
1 140164009678592 140164001285888     <--- For the first thread created (i = 2, then decremented)
0 140164009678592 140163992893184     <--- For the second thread created(i = 1, then decremented)
0 140164001285888 140163984500480     <--- For the third thread created(i = 1, then decremented)

The first column above is the value of 'i' (after it was decremented), the second column being the TID returned by pthread_self(), and lastly the third column is the TID returned by pthread_create(). You can see from the first thread above, that the TID is the same as the value returned by pthread_self() in the third thread.

P.S. Here's the code I test with, which btw, will generate warnings on an x86-64 system:

Code:

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

void* function(void* num)
{
    pthread_t tid;

    int i = (int) num;

    while (i > 0)
    {
        --i;

        pthread_create(&tid, NULL, function, (void*) i);

        printf("%d %lu %lu\n", i, pthread_self(), tid);
    }

    pthread_exit(NULL);
}

int main()
{
    pthread_t tid;

    pthread_create(&tid, NULL, function, (void*) 2);

    printf("  %lu %lu\n", pthread_self(), tid);

    sleep(1);

    return 0;
}

mcy · 05-07-2012, 06:22 AM

Thank u very much but in my case it prints the following:
2 1099352384 1109842240
1 1109842240 1120332096
0 1120332096 1130821952
0 1109842240 1120332096
1 1099352384 1109842240
0 1109842240 1120332096
0 1099352384 1109842240
and i cant understand why it prints 1109842240 1120332096 for all the iterations.. i mean the second number shouldn't be different each time? :/

dwhitney67 · 05-07-2012, 06:39 AM

If you are still using your original program, then perhaps you need to change the "%d" format in the printf() to be "%u". If you are on an x86-64 system, then change the format to be "%lu" as I did in the program I included earlier.

mcy · 05-07-2012, 06:47 AM

i think the reason that in your case it prints different number its because you remove pthread_join because i did the same thing and now i take different tid for each iteration. I still don't understand what is happening :/

dwhitney67 · 05-07-2012, 06:55 AM

Quote:

Originally Posted by mcy

i think the reason that in your case it prints different number its because you remove pthread_join because i did the same thing and now i take different tid for each iteration. I still don't understand what is happening :/

Yes, I took out the pthread_join()... and sorry, I forgot to replace it with a pthread_detach(). Here's the revised function:

Code:

void* function(void* num)
{
    ...

    while (i > 0)
    {
        ...
    }

    pthread_detach(pthread_self());

    return NULL;
}

In case you are wondering what the pthread_detach() does, here's the man-page description:

Code:

The pthread_detach() function  marks  the thread identified by thread as detached.
When a detached thread terminates, its resources are automatically released back
to the system without the need for another thread to join with the terminated thread.

mcy · 05-07-2012, 07:04 AM

ok thank u so much!

sorry for being tiring but please can u tell me what happens in case i have join? why tids remain the same? thank u again!

dwhitney67 · 05-07-2012, 08:35 AM

Quote:

Originally Posted by mcy

ok thank u so much!

sorry for being tiring but please can u tell me what happens in case i have join? why tids remain the same? thank u again!

I am not fully versed on the inner details of the pthread behavior, but I do know that pthread_join() will block the calling thread until the child-thread has terminated. It's possible that the system may reuse a TID previously assigned to a terminated thread, but I do not have any evidence to back this theory. Suffice to say, each thread that is currently running will have a unique TID.

mcy · 05-07-2012, 08:37 AM

ok thank u very much!!