Multiple execution of same thread subroutine on commenting pthread_join for that thread [duplicate]

Question

I am new to threading. Here if I comment pthread_join( thread1, NULL) then in the output sometimes I get

    Thread2
    Thread1
    Thread1

I am not able to understand why Thread1 trace is coming twice and what is the exact functionality of pthread_join.

Also, please refer some tutorial on threading concepts for beginners.

    void *print_message_function( void *ptr );
    main()
    {
            pthread_t thread1, thread2;
            char *message1 = "Thread 1";
            char *message2 = "Thread 2";
            int  iret1, iret2;
            iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);
            iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
            pthread_join( thread1, NULL);

            pthread_join( thread2, NULL); 

            printf("Thread 1 returns: %d\n",iret1);
            printf("Thread 2 returns: %d\n",iret2);
            exit(0);
    }

    void *print_message_function( void *ptr )
    {
            char *message;
            message = (char *) ptr;
            printf("%s \n", message);
    }

Answer 1

If I am getting these results, first of all I would do the following:

1) Instead of below lines,

iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);

iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
pthread_join( thread1, NULL);

pthread_join( thread2, NULL); 

replace it with,

iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);

pthread_join( thread1, NULL);
iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);

pthread_join( thread2, NULL); 

and see what is the result.

2) Inside your thread function, you need to call pthread_exit("Exit"); This is a proper way to exit from the thread function. Do it at the end of function.

void *print_message_function( void *ptr )
{
        char *message;
        message = (char *) ptr;
        printf("%s \n", message);
        pthread_exit("Exit"); 
    }

If you are doing it this way, ideally you should not face any problem. In every case, i am assuming you are compiling your program using gcc -D_REENTRANT -o threadex threadex.c -lpthread

This is not the final solution. If it is going well, then we can proceed to next step of starting both the threads at a time.

Please share the feedback after incorporating these changes.

Answer 2

Perhaps the output buffer is not flushing correctly. I've encountered a very similar issue when doing multithreading and piping the output to a file--sometimes output would appear twice. Try adding this line to your main function:

setvbuf(stdout, NULL, _IONBF, 0);

This will force the output buffer to be flushed on each write.