# C++ named requirements: Mutex

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The Mutex requirements extends the Lockable requirements to include inter-thread synchronization.

### Requirements

- Lockable
- DefaultConstructible
- Destructible
- not copyable
- not movable

For an object `m` of Mutex type:

- The expression m.lock() has the following properties

  - Behaves as an atomic operation.
  - Blocks the calling thread until exclusive ownership of the mutex can be obtained.
  - Prior m.unlock() operations on the same mutex synchronize-with this lock operation (equivalent to release-acquire std::memory_order)
  - The behavior is undefined if the calling thread already owns the mutex (except if m is std::recursive_mutex or std::recursive_timed_mutex)
  - Exception of type std::system_error may be thrown on errors, with the following error codes:
    - std::errc::operation_not_permitted if the calling thread does not have required privileges
    - std::errc::resource_deadlock_would_occur if the implementation detects that this operation would lead to deadlock

    - std::errc::device_or_resource_busy if the mutex is already locked

(until C++17)

- The expression m.try_lock() has the following properties

  - Behaves as an atomic operation.
  - Attempts to obtain exclusive ownership of the mutex for the calling thread without blocking. If ownership is not obtained, returns immediately. The function is allowed to spuriously fail and return even if the mutex is not currently owned by another thread.
  - If `try_lock()` succeeds, prior `unlock()` operations on the same object synchronize-with this operation (equivalent to release-acquire std::memory_order). `lock()` does not synchronize with a failed `try_lock()`
  - Does not throw exceptions.

- The expression m.unlock() has the following properties

  - Behaves as an atomic operation.
  - Releases the calling thread's ownership of the mutex and synchronizes-with the subsequent successful lock operations on the same object.
  - The behavior is undefined if the calling thread does not own the mutex.
  - Does not throw exceptions.

- All lock and unlock operations on a single mutex occur in a single total order that can be viewed as modification order of an atomic variable: the order is specific to this individual mutex.

### Library types

The following standard library types satisfy Mutex:

- std::mutex
- std::recursive_mutex
- std::timed_mutex
- std::recursive_timed_mutex
- std::shared_mutex

### See also

- Thread support library
- Lockable
- TimedMutex

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