C Threads

programming C MOC

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For processor-intensive programs, multithreading can run tasks concurrently.

A thread is an independent stream of instructions with a shared resource space for communications and data usage between threads, unlike processes, which run independently. Threads run in a group in parallel, not like forking.

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
 
void *outputMsg()
{
    sleep(1);
    puts("Hello from the thread!");
    return NULL;
}
 
int main()
{
    pthread_t threadId; // Given integer value of id
    printf("Starting thread id: %ld\n", threadId);
    
    pthread_create(&threadId, NULL, outputMsg, NULL);
    // Creates the new thread with no attributes on
	// function outputMsg with no attributes
	
    pthread_join(threadId, NULL);
    // Join thread to main thread
    
    sleep(1);
    printf("Exiting...\n");
    return 0;
}

Mutexes

A mutex helps solve race conditions with threads by setting a lock on some shared resource that can only be unlocked by that thread.

This is necessary for threads that need to happen in one at a time. A mutex is not needed for threads trying to crack a hash,

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
 
pthread_t threadId[5]; // Allows for 5 threads
pthread_mutex_t lock;
int counter = 0;
 
void* outputMsg(void *arg) {
    pthread_mutex_lock(&lock); // Locks code
    printf("Task %d start\n", ++counter);
    sleep(1);
    printf("Task %d finish\n\n", counter);
    pthread_mutex_unlock(&lock); // Unlocks code
    return NULL;
}
 
int main() {
    pthread_mutex_init(&lock, NULL); // Initialize lock
 
    for (int i = 0; i < 5; i++) {
        pthread_create(&(threadId[i]), NULL, outputMsg, NULL);
    } // Create 5 threads
 
    for (int i = 0; i < 5; i++) {
        pthread_join(threadId[i], NULL);
    } // Join 5 threads to main process
 
    pthread_mutex_destroy(&lock); // Remove lock
 
    return 0;
}