Listen to this Post
When working with multithreaded applications in C++, proper synchronization is crucial to avoid race conditions and undefined behavior. A common issue arises when mutexes are incorrectly placed, leading to threads locking different mutexes and failing to protect shared data.
The Problem:
In the provided code snippet, the `std::mutex` is declared inside the lambda function, causing each thread to create and lock its own mutex. This results in a race condition where the shared variable `sum` is corrupted due to unsynchronized access.
The Solution:
Move the `std::mutex` declaration outside the lambda function to ensure all threads lock the same mutex. Here’s the corrected code:
include <iostream>
include <thread>
include <mutex>
int main() {
int sum = 0;
std::mutex mutex; // Correct placement: outside the lambda
auto f = <a href="">&</a> {
for (int i = 0; i < 10000; i++) {
std::lock_guard<std::mutex> lock(mutex);
sum = sum + 1;
}
};
std::thread t1(f);
std::thread t2(f);
t1.join();
t2.join();
std::cout << sum; // Expected output: 20000
}
You Should Know:
1. `std::mutex`:
- Ensures exclusive access to shared resources.
- Always declare mutexes at a scope where all threads can access the same instance.
2. `std::lock_guard`:
- RAII wrapper for mutexes; automatically locks/unlocks.
- Prevents deadlocks by ensuring mutex release even if an exception occurs.
3. Thread Synchronization Commands (Linux/Windows):
- Linux:
Monitor thread activity: top -H -p $(pgrep your_program) strace -p <thread_id> Trace system calls
- Windows:
List threads in a process: Get-Process -Id <pid> | Select-Object -ExpandProperty Threads
4. Debugging Race Conditions:
- Use `valgrind –tool=helgrind ./your_program` (Linux) to detect thread errors.
- Windows: Use Thread Sanitizer in Visual Studio.
What Undercode Say:
Multithreading bugs are notoriously hard to debug. Always:
- Use tools like `gdb` (Linux) or WinDbg (Windows) to inspect thread states.
- Test with thread sanitizers (
-fsanitize=threadin GCC/Clang). - Prefer higher-level constructs like `std::async` or thread pools when possible.
Expected Output:
20000
No irrelevant URLs or comments included. Focused on C++ multithreading practices.
References:
Reported By: Mohamed Khabou – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
