[thread.mutex.recursive]

# 32 Concurrency support library [[thread]](./#thread)

## 32.6 Mutual exclusion [[thread.mutex]](thread.mutex#recursive)

### 32.6.4 Mutex requirements [[thread.mutex.requirements]](thread.mutex.requirements#thread.mutex.recursive)

#### 32.6.4.2 Mutex types [[thread.mutex.requirements.mutex]](thread.mutex.requirements.mutex#thread.mutex.recursive)

#### 32.6.4.2.3 Class recursive_mutex [thread.mutex.recursive]

[ð](#lib:recursive_mutex)

namespace std {class recursive_mutex {public: recursive_mutex(); ~recursive_mutex();

 recursive_mutex(const recursive_mutex&) = delete;
 recursive_mutex& operator=(const recursive_mutex&) = delete; void lock(); bool try_lock() noexcept; void unlock(); using native_handle_type = *implementation-defined*; // see [[thread.req.native]](thread.req.native "32.2.3 Native handles") native_handle_type native_handle(); // see [[thread.req.native]](thread.req.native "32.2.3 Native handles")};}

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7566)

The class recursive_mutex provides a recursive mutex with exclusive ownership
semantics[.](#1.sentence-1)

If one thread owns a recursive_mutex object, attempts by another
thread to acquire ownership of that object will fail (for try_lock()) or block
(for lock()) until the first thread has completely released ownership[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7573)

The class recursive_mutex meets
all of the mutex requirements ([[thread.mutex.requirements]](thread.mutex.requirements "32.6.4 Mutex requirements"))[.](#2.sentence-1)

It is a standard-layout class ([[class.prop]](class.prop "11.2 Properties of classes"))[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7578)

A thread that owns a recursive_mutex object may acquire additional levels of
ownership by calling lock() or try_lock() on that object[.](#3.sentence-1)

It is
unspecified how many levels of ownership may be acquired by a single thread[.](#3.sentence-2)

If a thread
has already acquired the maximum level of ownership for a recursive_mutex object, additional calls to try_lock() fail, and additional calls tolock() throw an exception of type system_error[.](#3.sentence-3)

A thread
shall call unlock() once for each level of ownership acquired by calls tolock() and try_lock()[.](#3.sentence-4)

Only when all levels of ownership have been
released may ownership be acquired by another thread[.](#3.sentence-5)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7589)

The behavior of a program is undefined if

- [(4.1)](#4.1)

it destroys a recursive_mutex object owned by any thread or

- [(4.2)](#4.2)

a thread terminates while owning a recursive_mutex object[.](#4.sentence-1)
