std::recursive_mutex::lock(3)

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 <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();
 }

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