std::recursive_mutex
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std::recursive_mutex
Member functions
Locking
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_mutexfor a period of time that starts when it successfully calls eitherlockortry_lock. During this period, the thread may make additional calls tolockortry_lock. The period of ownership ends when the thread makes a matching number of calls tounlock. - When a thread owns a
recursive_mutex, all other threads will block (for calls tolock) or receive a false return value (fortry_lock) if they attempt to claim ownership of therecursive_mutex. - The maximum number of times that a
recursive_mutexmay be locked is unspecified, but after that number is reached, calls tolockwill throw std::system_error and calls totry_lockwill 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.
[edit] Member types
Member type
Definition
[edit] Member functions
Locking
Native handle
[edit] 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 <mutex> #include <thread> 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