33 Concurrency support library [thread]

33.6 Mutual exclusion [thread.mutex]

33.6.4 Mutex requirements [thread.mutex.requirements]

33.6.4.4 Shared mutex types [thread.sharedmutex.requirements]

33.6.4.4.1 General [thread.sharedmutex.requirements.general]

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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]
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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.
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The expression m.lock_shared() is well-formed and has the following semantics:
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Preconditions: The calling thread has no ownership of the mutex.
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Effects: Blocks the calling thread until shared ownership of the mutex can be obtained for the calling thread.
If an exception is thrown then a shared lock has not been acquired for the current thread.
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Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
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Postconditions: The calling thread has a shared lock on the mutex.
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Return type: void.
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Throws: system_error when an exception is required ([thread.req.exception]).
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Error conditions:
  • (10.1)
    operation_not_permitted — if the thread does not have the privilege to perform the operation.
  • (10.2)
    resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.
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The expression m.unlock_shared() is well-formed and has the following semantics:
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Preconditions: The calling thread holds a shared lock on the mutex.
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Effects: Releases a shared lock on the mutex held by the calling thread.
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Return type: void.
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Synchronization: This operation synchronizes with subsequent lock() operations that obtain ownership on the same object.
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Throws: Nothing.
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The expression m.try_lock_shared() is well-formed and has the following semantics:
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Preconditions: The calling thread has no ownership of the mutex.
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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.
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Synchronization: If try_lock_shared() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
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Return type: bool.
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Returns: true if the shared lock was acquired, otherwise false.
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Throws: Nothing.

33.6.4.4.2 Class shared_mutex [thread.sharedmutex.class]

🔗
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] }; }
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The class shared_mutex provides a non-recursive mutex with shared ownership semantics.
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The class shared_mutex meets all of the shared mutex requirements ([thread.sharedmutex.requirements]).
It is a standard-layout class ([class.prop]).
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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.
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shared_mutex may be a synonym for shared_timed_mutex.