# std::mutex::unlock

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< cpp‎ | thread‎ | mutex

void unlock(); |  | (since C++11)

Unlocks the mutex.

The mutex must be locked by the current thread of execution, otherwise, the behavior is undefined.

This operation synchronizes-with (as defined in std::memory_order) any subsequent lock operation that obtains ownership of the same mutex.

### Parameters

(none)

### Return value

(none)

### Exceptions

(none)

### Notes

`unlock()` is usually not called directly: std::unique_lock and std::lock_guard are used to manage exclusive locking.

### Example

This example shows how `lock` and `unlock` can be used to protect shared data.

Run this code

    #include <iostream>
    #include <chrono>
    #include <thread>
    #include <mutex>

    int g_num = 0;  // protected by g_num_mutex
    std::mutex g_num_mutex;

    void slow_increment(int id)
    {
        for (int i = 0; i < 3; ++i) {
            g_num_mutex.lock();
            ++g_num;
            std::cout << id << " => " << g_num << '\n';
            g_num_mutex.unlock();

            std::this_thread::sleep_for(std::chrono::seconds(1));
        }
    }

    int main()
    {
        std::thread t1(slow_increment, 0);
        std::thread t2(slow_increment, 1);
        t1.join();
        t2.join();
    }

Possible output:

    0 => 1
    1 => 2
    0 => 3
    1 => 4
    0 => 5
    1 => 6

### See also

lock | locks the mutex, blocks if the mutex is not available (public member function)
try_lock | tries to lock the mutex, returns if the mutex is not available (public member function)
C documentation for mtx_unlock

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