Data Race implementing Atomic Operations in C - Unexpected Output in Multithreaded Environment
I'm refactoring my project and I'm migrating some code and I'm sure I'm missing something obvious here, but I'm working with a peculiar scenario with atomic operations in my multithreaded C program. I am using GCC 11.2 on Ubuntu 22.04 and I have a shared integer that multiple threads are incrementing. I'm relying on `__atomic_fetch_add` to perform the increments safely. However, I am observing unexpected results which suggest that the increments might not be working as intended. Here's a simplified version of my code: ```c #include <stdio.h> #include <stdlib.h> #include <pthread.h> #include <stdatomic.h> #define NUM_THREADS 10 #define INCREMENTS_PER_THREAD 1000 atomic_int shared_counter = 0; void* increment_counter(void* arg) { for (int i = 0; i < INCREMENTS_PER_THREAD; i++) { __atomic_fetch_add(&shared_counter, 1, __ATOMIC_SEQ_CST); } return NULL; } int main(void) { pthread_t threads[NUM_THREADS]; for (int i = 0; i < NUM_THREADS; i++) { pthread_create(&threads[i], NULL, increment_counter, NULL); } for (int i = 0; i < NUM_THREADS; i++) { pthread_join(threads[i], NULL); } printf("Final counter value: %d\n", shared_counter); return 0; } ``` When I run this code, I expect the final counter value to be 10,000 (10 threads * 1,000 increments each). However, I often see values like 9,982 or even lower, which indicates that some increments are being lost. I've verified that no other part of the code is modifying `shared_counter`, but I still need to pinpoint the question. I tried changing the memory order from `__ATOMIC_SEQ_CST` to `__ATOMIC_RELAXED`, thinking it might be causing some overhead, but the scenario continues. Additionally, I've ensured that my compiler optimizations are set to `-O0` during debugging to see if optimizations are interfering. Any insights or suggestions on what might be going wrong here? Is there something about the thread scheduling or atomic operations that I might be overlooking? Thank you! What am I doing wrong? I'd love to hear your thoughts on this. I recently upgraded to C stable.