std::recursive_mutex::lock

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

Parameters

(none)

Return value

(none)

Exceptions

Throws std::system_error when errors occur, including errors from the underlying operating system that would prevent lock

Notes

lock() is usually not called directly: std::unique_lock, std::scoped_lock, and std::lock_guard

Example

#include <chrono>
#include <iostream>
#include <mutex>
#include <thread>
 
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();
}

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