Deadlock: the four conditions and how to break them

A C program that deadlocks on purpose, the four necessary conditions, and the practical fix.

12 min read · Lesson 4 of 6 · Pro

A deadlock you can run

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

pthread_mutex_t A = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t B = PTHREAD_MUTEX_INITIALIZER;

void *worker1(void *arg) {
    (void)arg;
    pthread_mutex_lock(&A);
    printf("worker1 holds A, wants B\n");
    sleep(1);
    pthread_mutex_lock(&B);
    printf("worker1 got both\n");
    pthread_mutex_unlock(&B);
    pthread_mutex_unlock(&A);
    return NULL;
}

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