Unexpected behavior when using `volatile` keyword with multi-threaded flag in C
I'm migrating some code and I'm attempting to set up I'm trying to configure I'm having a hard time understanding I keep running into I'm encountering an unexpected behavior when using the `volatile` keyword in a multi-threaded C application. I have a flag variable that is shared between two threads, and I marked it as `volatile` to ensure that the compiler does not optimize accesses to it. However, even with `volatile`, I occasionally observe inconsistent results when checking this flag. Here’s a simplified version of my code: ```c #include <stdio.h> #include <pthread.h> #include <unistd.h> volatile int flag = 0; void* thread_func(void* arg) { sleep(1); // Simulate work flag = 1; // Set flag return NULL; } int main() { pthread_t thread; pthread_create(&thread, NULL, thread_func, NULL); while (flag == 0) { // Busy-wait } printf("Flag is set to 1\n"); pthread_join(thread, NULL); return 0; } ``` I expected that once the `flag` variable is set by the thread, the main thread would see this change immediately due to the `volatile` keyword. However, I sometimes find that the main thread continues to loop, indicating that it hasn't seen the updated value of `flag`. I'm using GCC 10.2 on Ubuntu 20.04, and I compile with `-pthread` flag. I've tried changing the order in which I check `flag`, and even added `sched_yield()` inside the loop, but the issue persists. Given that `volatile` is supposed to prevent optimizations, I wonder if there's something deeper at play here regarding memory visibility between threads, or if I’m missing a synchronization primitive. Any insights into what might be causing this behavior, and how I could fix it? This is part of a larger web app I'm building. I'm working with C in a Docker container on macOS. This is part of a larger REST API I'm building. This is happening in both development and production on macOS. I appreciate any insights! Is there a simpler solution I'm overlooking?