32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.3 Mutex requirements [thread.mutex.requirements]

32.5.3.4 Shared mutex types [thread.sharedmutex.requirements]

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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.
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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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Postconditions: The calling thread has a shared lock on the mutex.
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Return type: void.
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Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
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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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Return type: bool.
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Returns: true if the shared ownership lock was acquired, false otherwise.
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Synchronization: If try_­lock_­shared() returns true, prior unlock() operations on the same object synchronize with this operation.
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Throws: Nothing.

32.5.3.4.1 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.