C mutex example
std::mutex is used to protect shared resources to prevent data competition. In the example, the automatic locking and unlocking of std::lock_guard is used to ensure multi-thread safety; 1. Use std::mutex and std::lock_guard to avoid the abnormal risks brought by manual management of locks; 2. Shared variables such as counters must be protected with mutex when modifying multi-threads; 3. RAII-style lock management is recommended to ensure exception safety; 4. To avoid deadlocks, multiple locks need to be acquired in a fixed order; 5. Any multi-threaded scenario to access shared resources should use mutex synchronization, and the final program correctly outputs Expected: 10000 and Actual: 10000.
In C multi-threaded programming, std::mutex is an important tool used to protect shared resources and prevent multiple threads from simultaneously accessing data. Here is a simple example of using std::mutex , showing how to use it to safely modify shared variables in multiple threads.
? Basic usage: protect shared counters
#include <iostream>
#include <thread>
#include <mutex>
#include <vector>
std::mutex mtx; // Define a mutex int counter = 0; // Shared resource: Counter void increment(int n) {
for (int i = 0; i < n; i) {
mtx.lock(); // Lock counter; // Access shared resources (critical area)
mtx.unlock(); // Unlock}
}
int main() {
const int num_threads = 10;
const int inc_per_thread = 1000;
std::vector<stdhread> threads;
// Create multiple threads for (int i = 0; i < num_threads; i) {
threads.emplace_back(increment, inc_per_thread);
}
// Wait for all threads to complete for (auto& t : threads) {
t.join();
}
std::cout << "Expected: " << num_threads * inc_per_thread << std::endl;
std::cout << "Actual: " << counter << std::endl;
return 0;
}✅ Output example:
Expected: 10000 Actual: 10000
Without mutex , the actual result is likely to be less than 10000, because multiple threads may read and write counter at the same time, resulting in race conditions.
?️ More recommended writing method: use std::lock_guard
Manually calling lock() and unlock() is prone to errors (such as exceptions that cause unlock to not be executed). It is recommended to use RAII -style std::lock_guard to automatically manage the life cycle of locks.
void increment(int n) {
for (int i = 0; i < n; i) {
std::lock_guard<std::mutex> lock(mtx); // Automatic locking, automatic unlocking of counter after the scope ends;
}
}This ensures that the lock is released correctly even if an exception occurs in the critical area.
? Key points summary:
-
std::mutexis used to protect shared data. - Never do time-consuming or potentially throw exceptions when holding a lock.
- Use
std::lock_guardorstd::unique_lockinstead of manuallock/unlock. - Avoid deadlocks: Multiple locks should be obtained in a fixed order.
? Tips: When to use mutex?
When you have multiple threads:
- Read and write the same global variable
- Modify the same container (such as
std::vector,std::map) - Shared I/O resources such as writing logs and files
You need to use mutex to synchronize access.
Basically that's it. It is simple but easy to ignore details, and using lock_guard is the key.
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