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.



Return value



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.


lock() is usually not called directly: std::unique_lock, std::scoped_lock, and std::lock_guard are used to manage exclusive locking.


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

#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) {
        // note, that the mutex also syncronizes the output
        std::cout << "id: " << id << ", g_num: " << g_num << '\n';
int main()
    std::thread t1{slow_increment, 0};
    std::thread t2{slow_increment, 1};

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

tries to lock the mutex, returns if the mutex is not available
(public member function)
unlocks the mutex
(public member function)
C documentation for mtx_lock

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