pthread_exit vs. return

The following minimal test case exhibits the behaviour you describe:

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

void *app1(void *x)
{
    sleep(1);
    pthread_exit(0);
}

int main()
{
    pthread_t t1;

    pthread_create(&t1, NULL, app1, NULL);
    pthread_join(t1, NULL);

    return 0;
}

valgrind --leak-check=full --show-reachable=yes shows 5 blocks allocated from functions called by pthread_exit() that is unfreed but still reachable at process exit. If the pthread_exit(0); is replaced by return 0;, the 5 blocks are not allocated.

However, if you test creating and joining large numbers of threads, you will find that the amount of unfreed memory in use at exit does not increase. This, and the fact that it is still reachable, indicates that you’re just seeing an oddity of the glibc implementation. Several glibc functions allocate memory with malloc() the first time they’re called, which they keep allocated for the remainder of the process lifetime. glibc doesn’t bother to free this memory at process exit, since it knows that the process is being torn down anyway – it’d just be a waste of CPU cycles.