👀 Views: 35 💬 Answers: 1 📅 Created: 2025-06-06

c multithreading volatile C

This might be a silly question, but I'm experiencing unexpected behavior when using the `volatile` keyword in a multi-threaded application on Ubuntu 20.04. I have a simple producer-consumer setup where a producer thread updates a shared variable, and a consumer thread reads it. I declared the shared variable as `volatile int shared_var;`, expecting it to ensure that the compiler does not optimize out accesses to this variable. However, the consumer thread sometimes reads stale values instead of the updated ones. Here's a simplified version of my code: ```c #include <stdio.h> #include <stdlib.h> #include <pthread.h> #include <unistd.h> volatile int shared_var = 0; void *producer(void *arg) { for (int i = 0; i < 10; i++) { shared_var = i; printf("Produced: %d\n", i); usleep(100000); // Sleep for 100 ms } return NULL; } void *consumer(void *arg) { int last_value = -1; while (last_value < 9) { if (shared_var != last_value) { last_value = shared_var; printf("Consumed: %d\n", last_value); } usleep(50); // Sleep for 50 ms } return NULL; } int main() { pthread_t prod, cons; pthread_create(&prod, NULL, producer, NULL); pthread_create(&cons, NULL, consumer, NULL); pthread_join(prod, NULL); pthread_join(cons, NULL); return 0; } ``` When I run this code, the consumer sometimes outputs numbers that it should not have seen yet (e.g., `Consumed: 5` before `Consumed: 4`). I've tried adding a mutex lock around the assignment in the producer and the read in the consumer, but that introduces additional complexity and can lead to deadlocks if not handled correctly. I also read about memory barriers and atomic variables, but I'm unsure how to implement them properly in this context. Could someone please help clarify how to appropriately use `volatile` with multi-threaded access, or suggest an alternative approach that ensures consistent reads and writes without introducing race conditions? I'm working on a web app that needs to handle this. I'd really appreciate any guidance on this.