NAME

std::recursive_mutex::lock - std::recursive_mutex::lock

Synopsis

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