32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.3 Mutex requirements [thread.mutex.requirements] Mutex types [thread.mutex.requirements.mutex] Class recursive_­mutex [thread.mutex.recursive]

namespace std {
  class 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]
    native_handle_type native_handle();                         // see [thread.req.native]
The class recursive_­mutex provides a recursive mutex with exclusive ownership semantics.
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.
The class recursive_­mutex meets all of the mutex requirements ([thread.mutex.requirements]).
It is a standard-layout class ([class.prop]).
A thread that owns a recursive_­mutex object may acquire additional levels of ownership by calling lock() or try_­lock() on that object.
It is unspecified how many levels of ownership may be acquired by a single thread.
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 to lock() throw an exception of type system_­error.
A thread shall call unlock() once for each level of ownership acquired by calls to lock() and try_­lock().
Only when all levels of ownership have been released may ownership be acquired by another thread.
The behavior of a program is undefined if:
  • it destroys a recursive_­mutex object owned by any thread or
  • a thread terminates while owning a recursive_­mutex object.