Multithreading - how to stop all threads if condition is met?

Question

EDIT : I cannot use mutex or sleep() or any type of waiting as we're fairly new to threads in C.

For my assignment I'm to make a bruteforce password cracker using threads. The idea is that I have the hash and salt of a password so I have use a combination of the alphabet A-Z, a-z, 0-9 and encrypt it using the crypt_r function and compare it with the hash and salt I have.

I created 2 threads, one that just checks for lowercase letters and the second one that checks all the letters of the alphabet.

My problem is that once the lowercase thread finds the password it gives it but the second thread continues running in the background.

How can I stop the second thread without using any global variables?

Here's how I go about the thread joins :

for (int i = 0; i < nbThread; ++i) {
    pthread_join(tabThread[i], NULL);
    pthread_join(tabThread2[i], NULL);
}

Thank you for your time and help!

Possible duplicate of [How to stop a running pthread thread?](http://stackoverflow.com/questions/18826853/how-to-stop-a-running-pthread-thread) — Jean-François Fabre, Oct 31 '16 at 14:49

score 1 · Answer 1 · answered Oct 31 '16 at 14:51

1

Well you have pthread_cancel which is the nice way of telling a thread that it's time to exit. The threads needs to be aware of the possibility of cancellation, and handle it appropriately.

And then you have the not-nice-at-all way of just killing the thread with pthread_killl. Note that using pthread_kill could lead to resource leaks.

answered Oct 31 '16 at 14:51

Some programmer dude

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Can I call pthread_cancel within the thread? I pass a struct to my thread but the struct doesn't contain a pointer to the thread. – John Livingston Oct 31 '16 at 14:55
@JohnLivingston If you have the `pthread_t` of the thread you want to cancel, then you can call it from anywhere. – Some programmer dude Oct 31 '16 at 14:58

score 0 · Accepted Answer · answered Oct 31 '16 at 15:14

You don't have to use pthread_join() calls unless you want to do something after the threads exit. It's a convenience function, not a necessity. You can let the main thread exit with pthread_exit() (without any pthead_join() calls) and the other two threads can continue until either of them succeeded. If either of your threads find the password then you can simply exit (using _exit() or _Exit()) the process and the other thread will naturally be terminated on process exit.

thank you ! in my thread I added a "if password is found -> pthread_exit(NULL)" and it seems to work. — John Livingston, Oct 31 '16 at 15:21

Multithreading - how to stop all threads if condition is met?

2 Answers2