std::recursive_mutex::lock

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 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;
        std::cout << id << " => " << g_num << '\n';
        g_num_mutex.unlock();
 
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }
}
 
int main()
{
    std::thread t1(slow_increment, 0);
    std::thread t2(slow_increment, 1);
    t1.join();
    t2.join();
}

Possible output:

0 => 1
1 => 2
0 => 3
1 => 4
0 => 5
1 => 6

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
recursive_mutex::recursive_mutex
recursive_mutex::~recursive_mutex
Locking
recursive_mutex::lock
recursive_mutex::try_lock
recursive_mutex::unlock
Native handle
recursive_mutex::native_handle