void lock();
(since C++17)

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

If lock is called by a thread that already owns the mutex in any mode (shared or exclusive), the behavior is undefined.

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.

Shared mutexes do not support direct transition from shared to unique ownership mode: the shared lock has to be relinquished with unlock_shared() before exclusive ownership may be obtained with lock(). boost::upgrade_mutex may be used for this purpose.


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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