std::recursive_mutex

 

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std::recursive_mutex

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

 

Defined in header <mutex>
class recursive_mutex;		(since C++11)

The recursive_mutex class is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads.

recursive_mutex offers exclusive, recursive ownership semantics:

• A calling thread owns a recursive_mutex for a period of time that starts when it successfully calls either lock or try_lock. During this period, the thread may make additional calls to lock or try_lock. The period of ownership ends when the thread makes a matching number of calls to unlock.
• When a thread owns a recursive_mutex, all other threads will block (for calls to lock) or receive a false return value (for try_lock) if they attempt to claim ownership of the recursive_mutex.
• The maximum number of times that a recursive_mutex may be locked is unspecified, but after that number is reached, calls to lock will throw std::system_error and calls to try_lock will return false.

The behavior of a program is undefined if a recursive_mutex is destroyed while still owned by some thread. The recursive_mutex class satisfies all requirements of Mutex and StandardLayoutType.

Member types

Member type	Definition
native_handle_type(not always present)	implementation-defined

Member functions

(constructor)	constructs the mutex (public member function)
(destructor)	destroys the mutex (public member function)
operator= [deleted]	not copy-assignable (public member function)
Locking
lock	locks the mutex, blocks if the mutex is not available (public member function)
try_lock	tries to lock the mutex, returns if the mutex is not available (public member function)
unlock	unlocks the mutex (public member function)
Native handle
native_handle	returns the underlying implementation-defined native handle object (public member function)

Example

one use case for recursive_mutex is protecting shared state in a class whose member functions may call each other

Run this code

#include <iostream>
#include <thread>
#include <mutex>
 
class X {
    std::recursive_mutex m;
    std::string shared;
  public:
    void fun1() {
      std::lock_guard<std::recursive_mutex> lk(m);
      shared = "fun1";
      std::cout << "in fun1, shared variable is now " << shared << '\n';
    }
    void fun2() {
      std::lock_guard<std::recursive_mutex> lk(m);
      shared = "fun2";
      std::cout << "in fun2, shared variable is now " << shared << '\n';
      fun1(); // recursive lock becomes useful here
      std::cout << "back in fun2, shared variable is " << shared << '\n';
    };
};
 
int main() 
{
    X x;
    std::thread t1(&X::fun1, &x);
    std::thread t2(&X::fun2, &x);
    t1.join();
    t2.join();
}

Possible output:

in fun1, shared variable is now fun1
in fun2, shared variable is now fun2
in fun1, shared variable is now fun1
back in fun2, shared variable is fun1

See also

mutex (C++11)

provides basic mutual exclusion facility (class)