Race Condition When Using `pthread_cond_wait` and `pthread_mutex_lock` in C
I've encountered a strange issue with I'm working through a tutorial and I'm trying to debug I'm working on a project and hit a roadblock..... I'm experiencing a race condition in my multithreaded C application that's using POSIX threads. I have a shared resource that's being accessed by multiple threads, and I implemented condition variables to manage access. The issue arises when one thread is waiting on a condition variable while another thread modifies the shared resource. Sometimes, it seems like the waiting thread doesn't get notified even though the condition is met, leading to unexpected behavior and deadlocks. Here's a simplified version of my code: ```c #include <stdio.h> #include <stdlib.h> #include <pthread.h> #define BUFFER_SIZE 5 int buffer[BUFFER_SIZE]; int count = 0; pthread_mutex_t mutex; pthread_cond_t cond_var; void* producer(void* arg) { for (int i = 0; i < 10; i++) { pthread_mutex_lock(&mutex); while (count == BUFFER_SIZE) { pthread_cond_wait(&cond_var, &mutex); } buffer[count++] = i; printf("Produced: %d\n", i); pthread_cond_signal(&cond_var); pthread_mutex_unlock(&mutex); } return NULL; } void* consumer(void* arg) { for (int i = 0; i < 10; i++) { pthread_mutex_lock(&mutex); while (count == 0) { pthread_cond_wait(&cond_var, &mutex); } int item = buffer[--count]; printf("Consumed: %d\n", item); pthread_cond_signal(&cond_var); pthread_mutex_unlock(&mutex); } return NULL; } int main() { pthread_t prod, cons; pthread_mutex_init(&mutex, NULL); pthread_cond_init(&cond_var, NULL); pthread_create(&prod, NULL, producer, NULL); pthread_create(&cons, NULL, consumer, NULL); pthread_join(prod, NULL); pthread_join(cons, NULL); pthread_mutex_destroy(&mutex); pthread_cond_destroy(&cond_var); return 0; } ``` I have ensured that the mutex is locked before accessing the shared buffer, and I unlock it after signaling the condition variable. However, I'm seeing occasional outputs where a consumer tries to consume items even when the buffer is empty, which shouldn't happen. I've tried adding additional debug prints to check the values of `count` before the wait calls, and they seem correct, but I suspect that the notification from `pthread_cond_signal` might not be waking up the waiting thread as expected. Are there known pitfalls with using condition variables in this way? Any suggestions would be appreciated, including adjustments to my locking strategy or condition variable usage. This is part of a larger service I'm building. Is there a better approach? What's the correct way to implement this? This is for a service running on Debian. This is my first time working with C 3.9. Any ideas what could be causing this? Hoping someone can shed some light on this.