First store the thread id
pthread_create(&thr, ...)
then later call
pthread_cancel(thr)
However, this not a recommended programming practice! It’s better to use an inter-thread communication mechanism like semaphores or messages to communicate to the thread that it should stop execution.
Note that pthread_kill(…) does not actually terminate the receiving thread, but instead delivers a signal to it, and it depends on the signal and signal handlers what happens.