[thread.sharedmutex.requirements]

The standard library types shared_mutex and shared_timed_mutex are shared mutex types.

Shared mutex types meet the requirements of mutex types ([thread.mutex.requirements.mutex]) and additionally meet the requirements set out below.

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

[Note 1:

The shared mutex types meet the Cpp17SharedLockable requirements ([thread.req.lockable.shared]).

— end note]

In addition to the exclusive lock ownership mode specified in [thread.mutex.requirements.mutex], shared mutex types provide a shared lock ownership mode.

Multiple execution agents can simultaneously hold a shared lock ownership of a shared mutex type.

But no execution agent holds a shared lock while another execution agent holds an exclusive lock on the same shared mutex type, and vice-versa.

The maximum number of execution agents which can share a shared lock on a single shared mutex type is unspecified, but is at least 10000.

If more than the maximum number of execution agents attempt to obtain a shared lock, the excess execution agents block until the number of shared locks are reduced below the maximum amount by other execution agents releasing their shared lock.

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

Postconditions: The calling thread has a shared lock on the mutex.

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

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

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

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

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

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

Throws: Nothing.

32.6.4.4.2 Class shared_mutex [thread.sharedmutex.class]

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namespace std { class shared_mutex { public: shared_mutex(); ~shared_mutex(); shared_mutex(const shared_mutex&) = delete; shared_mutex& operator=(const shared_mutex&) = delete; / exclusive ownership void lock(); / blocking bool try_lock(); void unlock(); / shared ownership void lock_shared(); / blocking bool try_lock_shared(); void unlock_shared(); using native_handle_type = implementation-defined; / see [thread.req.native] native_handle_type native_handle(); / see [thread.req.native] }; }

The class shared_mutex provides a non-recursive mutex with shared ownership semantics.

The class shared_mutex meets all of the shared mutex requirements ([thread.sharedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if

(3.1)
it destroys a shared_mutex object owned by any thread,
(3.2)
a thread attempts to recursively gain any ownership of a shared_mutex, or
(3.3)
a thread terminates while possessing any ownership of a shared_mutex.

shared_mutex may be a synonym for shared_timed_mutex.