NAME

std::recursive_mutex - std::recursive_mutex

Synopsis

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= not copy-assignable
[deleted] (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