I’m using Boost.Interprocess to implement interprocess synchronization with a named mutex and shared memory. I have two processes that need to coordinate access to a shared variable. Both processes call find_or_construct to create a mutex and a shared integer variable. However, I encounter a problem where both threads sometimes enter the critical section despite using locks.

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/sync/interprocess_mutex.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <iostream>

namespace ipc = boost::interprocess;

void process(const char* shared_memory_name, const char* mutex_name, const char* var_name) {
    ipc::managed_shared_memory segment(ipc::open_or_create, shared_memory_name, 65536);

    ipc::interprocess_mutex *mutex = segment.find_or_construct<ipc::interprocess_mutex>(mutex_name)();
    auto value = segment.find_or_construct<int64_t>(var_name)(-1);

    while (true) {
        ipc::scoped_lock<ipc::interprocess_mutex> lock(*mutex, ipc::try_to_lock);
        if (lock) {
            if (*value < 0) {
                *value = 5;
            }
            break;
        }
    }

    std::cout << "Process completed with value: " << *value << std::endl;
}

int main() {
    const char* shared_memory_name = "SharedMemory";
    const char* mutex_name = "MyMutex";
    const char* var_name = "MyVar";

    process(shared_memory_name, mutex_name, var_name);

    return 0;
}

The issue is that sometimes both threads enter the critical section where value < 0 should be checked and modified. This results in both threads setting the variable even though the lock is supposed to ensure exclusive access.

Could this issue be due to memory ordering, where one thread’s update to value is not visible to the other thread when it acquires the lock?
Is there something wrong with my implementation of the locking mechanism using Boost.Interprocess, or is there a better way to ensure exclusive access?
Any insights or suggestions to resolve this issue would be greatly appreciated!