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

constructs the mutex
(public member function)
destroys the mutex
(public member function)
not copy-assignable
(public member function)
locks the mutex, blocks if the mutex is not available
(public member function)
tries to lock the mutex, returns if the mutex is not available
(public member function)
unlocks the mutex
(public member function)
Native handle
returns the underlying implementation-defined native handle object
(public member function)


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

#include <iostream>
#include <thread>
#include <mutex>
class X {
    std::recursive_mutex m;
    std::string shared;
    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);

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

provides basic mutual exclusion facility

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