32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.4 Mutex requirements [thread.mutex.requirements]

32.5.4.3 Timed mutex types [thread.timedmutex.requirements]

32.5.4.3.1 General [thread.timedmutex.requirements.general]

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The timed mutex types are the standard library types timed_­mutex, recursive_­timed_­mutex, and shared_­timed_­mutex.
They meet the requirements set out below.
In this description, m denotes an object of a mutex type, rel_­time denotes an object of an instantiation of duration, and abs_­time denotes an object of an instantiation of time_­point.
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The timed mutex types meet the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
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The expression m.try_­lock_­for(rel_­time) is well-formed and has the following semantics:
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Preconditions: If m is of type timed_­mutex or shared_­timed_­mutex, the calling thread does not own the mutex.
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Effects: The function attempts to obtain ownership of the mutex within the relative timeout ([thread.req.timing]) specified by rel_­time.
If the time specified by rel_­time is less than or equal to rel_­time.zero(), the function attempts to obtain ownership without blocking (as if by calling try_­lock()).
The function returns within the timeout specified by rel_­time only if it has obtained ownership of the mutex object.
[Note 1:
As with try_­lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so.
— end note]
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Return type: bool.
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Returns: true if ownership was obtained, otherwise false.
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Synchronization: If try_­lock_­for() returns true, prior unlock() operations on the same object synchronize with this operation.
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Throws: Timeout-related exceptions ([thread.req.timing]).
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The expression m.try_­lock_­until(abs_­time) is well-formed and has the following semantics:
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Preconditions: If m is of type timed_­mutex or shared_­timed_­mutex, the calling thread does not own the mutex.
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Effects: The function attempts to obtain ownership of the mutex.
If abs_­time has already passed, the function attempts to obtain ownership without blocking (as if by calling try_­lock()).
The function returns before the absolute timeout ([thread.req.timing]) specified by abs_­time only if it has obtained ownership of the mutex object.
[Note 2:
As with try_­lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so.
— end note]
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Return type: bool.
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Returns: true if ownership was obtained, otherwise false.
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Synchronization: If try_­lock_­until() returns true, prior unlock() operations on the same object synchronize with this operation.
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Throws: Timeout-related exceptions ([thread.req.timing]).

32.5.4.3.2 Class timed_­mutex [thread.timedmutex.class]

namespace std { class timed_mutex { public: timed_mutex(); ~timed_mutex(); timed_mutex(const timed_mutex&) = delete; timed_mutex& operator=(const timed_mutex&) = delete; void lock(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); 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 timed_­mutex provides a non-recursive mutex with exclusive ownership semantics.
If one thread owns a timed_­mutex object, attempts by another thread to acquire ownership of that object will fail (for try_­lock()) or block (for lock(), try_­lock_­for(), and try_­lock_­until()) until the owning thread has released ownership with a call to unlock() or the call to try_­lock_­for() or try_­lock_­until() times out (having failed to obtain ownership).
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The class timed_­mutex meets all of the timed mutex requirements ([thread.timedmutex.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 timed_­mutex object owned by any thread,
  • (3.2)
    a thread that owns a timed_­mutex object calls lock(), try_­lock(), try_­lock_­for(), or try_­lock_­until() on that object, or
  • (3.3)
    a thread terminates while owning a timed_­mutex object.

32.5.4.3.3 Class recursive_­timed_­mutex [thread.timedmutex.recursive]

namespace std { class recursive_timed_mutex { public: recursive_timed_mutex(); ~recursive_timed_mutex(); recursive_timed_mutex(const recursive_timed_mutex&) = delete; recursive_timed_mutex& operator=(const recursive_timed_mutex&) = delete; void lock(); // blocking bool try_lock() noexcept; template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); 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 recursive_­timed_­mutex provides a recursive mutex with exclusive ownership semantics.
If one thread owns a recursive_­timed_­mutex object, attempts by another thread to acquire ownership of that object will fail (for try_­lock()) or block (for lock(), try_­lock_­for(), and try_­lock_­until()) until the owning thread has completely released ownership or the call to try_­lock_­for() or try_­lock_­until() times out (having failed to obtain ownership).
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The class recursive_­timed_­mutex meets all of the timed mutex requirements ([thread.timedmutex.requirements]).
It is a standard-layout class ([class.prop]).
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A thread that owns a recursive_­timed_­mutex object may acquire additional levels of ownership by calling lock(), try_­lock(), try_­lock_­for(), or try_­lock_­until() on that object.
It is unspecified how many levels of ownership may be acquired by a single thread.
If a thread has already acquired the maximum level of ownership for a recursive_­timed_­mutex object, additional calls to try_­lock(), try_­lock_­for(), or try_­lock_­until() fail, and additional calls to lock() throw an exception of type system_­error.
A thread shall call unlock() once for each level of ownership acquired by calls to lock(), try_­lock(), try_­lock_­for(), and try_­lock_­until().
Only when all levels of ownership have been released may ownership of the object be acquired by another thread.
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The behavior of a program is undefined if:
  • (4.1)
    it destroys a recursive_­timed_­mutex object owned by any thread, or
  • (4.2)
    a thread terminates while owning a recursive_­timed_­mutex object.