Multithreading with pthreads

POSIX threads (pthread) let a C program run code concurrently on multiple threads:

#include <pthread.h>

void *printNumbers(void *arg) {
    for (int i = 0; i < 3; i++) {
        printf("Thread: %d\n", i);
    }
    return NULL;
}

int main(void) {
    pthread_t thread;
    pthread_create(&thread, NULL, printNumbers, NULL);
    pthread_join(thread, NULL); // wait for it to finish
    return 0;
}

gcc program.c -o program -lpthread

A pthread_mutex_t protects shared data from being modified by two threads at the same instant — exactly the same race-condition problem Java's synchronized solves, just with an explicit lock/unlock instead of a keyword:

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

void increment(int *counter) {
    pthread_mutex_lock(&lock);
    (*counter)++;
    pthread_mutex_unlock(&lock);
}

Common mistakes

• Forgetting pthread_join — the main thread can exit (ending the whole process) before a spawned thread finishes its work.