33 Concurrency support library [thread]

33.6 Mutual exclusion [thread.mutex]

33.6.4 Mutex requirements [thread.mutex.requirements]

33.6.4.2 Mutex types [thread.mutex.requirements.mutex]

33.6.4.2.1 General [thread.mutex.requirements.mutex.general]

The mutex types are the standard library types mutex, recursive_mutex, timed_mutex, recursive_timed_mutex, shared_mutex, and shared_timed_mutex.

They meet the requirements set out in [thread.mutex.requirements.mutex].

In this description, m denotes an object of a mutex type.

[Note 1:

The mutex types meet the Cpp17Lockable requirements ([thread.req.lockable.req]).

— end note]

The mutex types meet Cpp17DefaultConstructible and Cpp17Destructible.

If initialization of an object of a mutex type fails, an exception of type system_error is thrown.

The mutex types are neither copyable nor movable.

The error conditions for error codes, if any, reported by member functions of the mutex types are as follows:

(3.1)
resource_unavailable_try_again — if any native handle type manipulated is not available.
(3.2)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(3.3)
invalid_argument — if any native handle type manipulated as part of mutex construction is incorrect.

The implementation provides lock and unlock operations, as described below.

For purposes of determining the existence of a data race, these behave as atomic operations ([intro.multithread]).

The lock and unlock operations on a single mutex appears to occur in a single total order.

[Note 2:

This can be viewed as the modification order ([intro.multithread]) of the mutex.

— end note]

[Note 3:

Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization can be used to ensure that mutex objects are initialized and visible to other threads.

— end note]

The expression m.lock() is well-formed and has the following semantics:

Preconditions: If m is of type mutex, timed_mutex, shared_mutex, or shared_timed_mutex, the calling thread does not own the mutex.

Effects: Blocks the calling thread until ownership of the mutex can be obtained for the calling thread.

Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Postconditions: The calling thread owns the mutex.

Return type: void.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(12.1)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(12.2)
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.

The expression m.try_lock() is well-formed and has the following semantics:

Preconditions: If m is of type mutex, timed_mutex, shared_mutex, or shared_timed_mutex, the calling thread does not own the mutex.

Effects: Attempts to obtain ownership of the mutex for the calling thread without blocking.

If ownership is not obtained, there is no effect and try_lock() immediately returns.

An implementation may fail to obtain the lock even if it is not held by any other thread.

[Note 4:

This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange ([atomics]).

— end note]

An implementation should ensure that try_lock() does not consistently return false in the absence of contending mutex acquisitions.

Synchronization: If try_lock() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

[Note 5:

Since lock() does not synchronize with a failed subsequent try_lock(), the visibility rules are weak enough that little would be known about the state after a failure, even in the absence of spurious failures.

— end note]

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Nothing.

The expression m.unlock() is well-formed and has the following semantics:

Preconditions: The calling thread owns the mutex.

Effects: Releases the calling thread's ownership of the mutex.

Return type: void.

Synchronization: This operation synchronizes with ([intro.multithread]) subsequent lock operations that obtain ownership on the same object.

Throws: Nothing.