👀 Views: 66 💬 Answers: 1 📅 Created: 2025-06-07

gcc multithreading pthreads optimization C

I can't seem to get I'm following best practices but I'm relatively new to this, so bear with me. I tried several approaches but none seem to work. I'm experiencing unexpected behavior in my multithreaded C application when compiling with GCC 11.2. The issue arises only when I enable optimization flags (`-O2` or higher). My application is using pthreads for thread management and relies heavily on shared data structures. When I run the program with optimization flags, I get inconsistent results, especially when accessing a shared counter. Here is a simplified version of my code: ```c #include <stdio.h> #include <stdlib.h> #include <pthread.h> #define NUM_THREADS 4 int shared_counter = 0; pthread_mutex_t mutex; void* increment_counter(void* arg) { for (int i = 0; i < 100000; i++) { pthread_mutex_lock(&mutex); shared_counter++; pthread_mutex_unlock(&mutex); } return NULL; } int main() { pthread_t threads[NUM_THREADS]; pthread_mutex_init(&mutex, NULL); 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); pthread_mutex_destroy(&mutex); return 0; } ``` When I compile this code with `gcc -o myprogram myprogram.c -lpthread -O2`, the final value of `shared_counter` is often less than 400000, which is unexpected since all threads should be incrementing it 100000 times each. However, if I compile without any optimization flags (just `gcc -o myprogram myprogram.c -lpthread`), it works as intended and consistently gives me 400000. I've tried adding `volatile` to the shared variable declaration, but it didn't resolve the issue. I've also read about potential issues with compiler optimizations in multithreaded environments but am unsure how to resolve this specifically with my code. Is there a known issue with GCC's optimization that affects multithreaded programs like mine, and how can I ensure that the value is calculated correctly? Any insights would be appreciated. Any help would be greatly appreciated! This is my first time working with C 3.10. Thanks in advance! I recently upgraded to C latest. Could someone point me to the right documentation? I'm working on a CLI tool that needs to handle this. Has anyone else encountered this?