Thread and Forks

Question

I am relatively new to threads and forks. So to understand them a bit better I have been writing simple programs. One of the little programs I have written two programs, one to print a counter on two processes, and another with two threads. What I noticed is that the fork prints the counters interlaced while the thread prints one thread's counter and then the others. So the thread is not so parallel, but behaves more serial Why is that? Am I doing something wrong?

Also, what exactly does pthread_join do? Even when I don't do pthread_join the program runs similarly.

Here is my code for the thread

void * thread1(void *a){
    int i =0;
for(i=0; i<100; i++)
    printf("Thread 1      %d\n",i);
}

void * thread2(void *b){
int i =0;
for(i=0; i<100; i++)
    printf("Thread 2    %d\n", i);
}

int main()
{
pthread_t tid1,tid2;
pthread_create(&tid1,NULL,thread1, NULL);
pthread_create(&tid2,NULL,thread2, NULL);
pthread_join(tid1,NULL);
pthread_join(tid2,NULL);
return 0;
}

And here is my code for fork

int main(void)
{
pid_t childPID;
childPID = fork();

if(childPID >= 0) // fork was successful
{
    if(childPID == 0) // child process
    {   int i;
    for(i=0; i<100;i++)
        printf("\n Child Process Counter : %d\n",i);

    }
    else //Parent process
    {
        int i;
        for(i=0; i<100;i++)
            printf("\n Parent Process Counter : %d\n",i);
    }
}
else // fork failed
{
    printf("\n Fork failed, quitting!!!!!!\n");
    return 1;
}

return 0;
}

EDIT: How can I make the threaded program behave more like the fork program? i.e. the counter prints interweave.

Answer 1

You are traveling down a bad road here. The lesson you should be learning is to not try and out think the OS scheduler. No matter what you do - processing schedules, priorities, or whatever knobs you turn - you cannot do it reliably.

You have backed your way into discovering the need for synchronization mechanisms - mutexes, semaphores, condition variables, thread barriers, etc. What you want to do is exactly why they exist and what you should use to accomplish your goals.

On your last question, pthread_join reclaims some resources from dead, joinable (i.e.not detached) threads and allows you to inspect any return variable from the expired thread. In your program they are mostly serving as a blocking mechanism. That is, main will block on those calls until the threads expire. Without the pthread_joins your main would end and the process would die, including the threads you created. If you don't want to join the threads and aren't doing anything useful in main then use pthread_exit in main as this will allow main to exit but the threads to continue processing.

Answer 2

First of all, a fork creates a second process while creating a thread creates a "dispatchable unit of work" within the same process.

Getting two different processes to be interleaved is usually a simple matter of letting the OS run. However, within a process you need to know more about how the OS chooses which of several threads to run.

You could probably, artificially, get the output from the threads to be interleaved by calling sleep for different times from each thread. That is, create thread A (code it to output one line, and then sleep for 100) then create thread B (code it to output one line and then sleep for 50, etc.)

I understand wanting to see how threads can run in parallel, similar to processes. But, is this a real requirement or just a "zoo" request?