Thread safety

Learn thread safety and data race concepts.

Thread safety

Thread safety means code works correctly when called by multiple threads at once. Unprotected access to shared data causes a data race.

A data race occurs when two threads access the same variable and at least one writes. The result is unpredictable: counter++ called 2000 times may not yield 2000. With a mutex you serialize access and prevent the race.

Data race

A data race is when concurrent access gives an ambiguous result. If two threads do counter++, some increments can be lost.

For example, two threads incrementing 1000 times each may give 1732 instead of 2000. With a mutex each increment becomes atomic and the result is always 2000. Compiler warnings and tools can detect races; but the safest way is to use a mutex.

Exercise Race-free counter

Write void *worker(void *arg) with for (int i = 0; i < 100; i++) { pthread_mutex_lock(&lock); counter++; pthread_mutex_unlock(&lock); }. Create two threads, join them and print printf("counter=%d\n", counter);. With the mutex the result is 200.

Show example solution

#include <stdio.h>
#include <pthread.h>

int counter = 0;
pthread_mutex_t lock;

void *worker(void *arg) {
    for (int i = 0; i < 100; i++) {
        pthread_mutex_lock(&lock);
        counter++;
        pthread_mutex_unlock(&lock);
    }
    return NULL;
}

int main(void) {
    pthread_mutex_init(&lock, NULL);
    pthread_t t1, t2;
    pthread_create(&t1, NULL, worker, NULL);
    pthread_create(&t2, NULL, worker, NULL);
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    printf("counter=%d\n", counter);
    return 0;
}

Test yourself

Match safety concepts

mutex

Prevents race
data race

Ambiguous access
lock

Acquires lock
counter++

Increment operation
How is a data race prevented?
- With fork
- With printf
- With a mutex
- With exit
Show answer

With a mutex — A mutex serializes shared access.

Critical region

pthread_mutex_lock(&____);
counter++;
pthread_mutex_unlock(&lock);

Show answer

pthread_mutex_lock(&lock);
counter++;
pthread_mutex_unlock(&lock);

Next lesson: Summary - Pthreads

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