# std::recursive_mutex::lock

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< cpp‎ | thread‎ | recursive mutex

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

Locks the mutex. If another thread has already locked the mutex, a call to `lock` will block execution until the lock is acquired.

A thread may call `lock` on a recursive mutex repeatedly. Ownership will only be released after the thread makes a matching number of calls to `unlock`.

The maximum number of levels of ownership is unspecified. An exception of type std::system_error will be thrown if this number is exceeded.

Prior unlock() operations on the same mutex synchronize-with (as defined in std::memory_order) this operation.

### Parameters

(none)

### Return value

(none)

### Exceptions

Throws std::system_error when errors occur, including errors from the underlying operating system that would prevent `lock` from meeting its specifications. The mutex is not locked in the case of any exception being thrown.

### Notes

`lock()` is usually not called directly: std::unique_lock, `std::scoped_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;
            // note, that the mutex also syncronizes the output
            std::cout << "id: " << id << ", g_num: " << g_num << '\n';
            g_num_mutex.unlock();

            std::this_thread::sleep_for(std::chrono::milliseconds(234));
        }
    }

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

Possible output:

    id: 0, g_num: 1
    id: 1, g_num: 2
    id: 1, g_num: 3
    id: 0, g_num: 4
    id: 0, g_num: 5
    id: 1, g_num: 6

### See also

try_lock | tries to lock the mutex, returns if the mutex is not available (public member function)
unlock | unlocks the mutex (public member function)

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