The race you cannot see in C

The same increment with and without a lock, run enough times to lose counts.

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

#define ROUNDS 200000

static long unsafe_total = 0;
static long safe_total = 0;
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

static void *bump(void *arg) {
    (void) arg;
    for (int i = 0; i < ROUNDS; i++) {
        unsafe_total++;
        pthread_mutex_lock(&lock);
        safe_total++;
        pthread_mutex_unlock(&lock);
    }
    return NULL;
}

int main(void) {
    pthread_t a, b;
    pthread_create(&a, NULL, bump, NULL);
    pthread_create(&b, NULL, bump, NULL);
    pthread_join(a, NULL);
    pthread_join(b, NULL);

    printf("expected %d\n", 2 * ROUNDS);
    printf("unsafe   %ld\n", unsafe_total);
    printf("safe     %ld\n", safe_total);
    return 0;
}

How it works

1. total++ is three operations, not one.
2. Two threads interleaving them lose updates.
3. The unlocked total comes out lower, and differently each run.

Keywords and builtins used here

• NULL
• bump
• for
• int
• long
• main
• return
• static
• void

The run, in numbers

Lines

32

Characters to type

644

Tokens

167

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 95 seconds.

Type this snippet

Step 2 of 4 in Locking, step 5 of 10 in Threads & concurrency.

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