[thread.sharedtimedmutex.requirements]

# 32 Concurrency support library [[thread]](./#thread)

## 32.6 Mutual exclusion [[thread.mutex]](thread.mutex#thread.sharedtimedmutex.requirements)

### 32.6.4 Mutex requirements [[thread.mutex.requirements]](thread.mutex.requirements#thread.sharedtimedmutex.requirements)

#### 32.6.4.5 Shared timed mutex types [thread.sharedtimedmutex.requirements]

#### [32.6.4.5.1](#general) General [[thread.sharedtimedmutex.requirements.general]](thread.sharedtimedmutex.requirements.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7987)

The standard library type shared_timed_mutex is a[*shared timed mutex type*](#def:shared_timed_mutex_type "32.6.4.5.1 General [thread.sharedtimedmutex.requirements.general]")[.](#general-1.sentence-1)

Shared timed mutex types meet the requirements of
timed mutex types ([[thread.timedmutex.requirements]](thread.timedmutex.requirements "32.6.4.3 Timed mutex types")),
shared mutex types ([[thread.sharedmutex.requirements]](thread.sharedmutex.requirements "32.6.4.4 Shared mutex types")), and additionally
meet the requirements set out below[.](#general-1.sentence-2)

In this description,m denotes an object of a shared timed mutex type,rel_time denotes an object of an instantiation ofduration ([[time.duration]](time.duration "30.5 Class template duration")), andabs_time denotes an object of an instantiation of[time_point](time.point "30.6 Class template time_­point [time.point]")[.](#general-1.sentence-3)

[*Note [1](#general-note-1)*:

The shared timed mutex types meet the [*Cpp17SharedTimedLockable*](thread.req.lockable.shared.timed#:Cpp17SharedTimedLockable "32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]") requirements ([[thread.req.lockable.shared.timed]](thread.req.lockable.shared.timed "32.2.5.6 Cpp17SharedTimedLockable requirements"))[.](#general-1.sentence-4)

â *end note*]

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8003)

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

[3](#general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8007)

*Preconditions*: The calling thread has no ownership of the mutex[.](#general-3.sentence-1)

[4](#general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8011)

*Effects*: Attempts to obtain
shared lock ownership for the calling thread within the relative
timeout ([[thread.req.timing]](thread.req.timing "32.2.4 Timing specifications")) specified by rel_time[.](#general-4.sentence-1)

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 callingtry_lock_shared())[.](#general-4.sentence-2)

The function returns within the timeout
specified by rel_time only if it has obtained shared ownership of the
mutex object[.](#general-4.sentence-3)

[*Note [2](#general-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[.](#general-4.sentence-4)

â *end note*]

If an exception is thrown then a shared lock has not been acquired for
the current thread[.](#general-4.sentence-5)

[5](#general-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8029)

*Synchronization*: If try_lock_shared_for() returns true, priorunlock() operations on the same object synchronize
with ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races")) this operation[.](#general-5.sentence-1)

[6](#general-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8035)

*Return type*: bool[.](#general-6.sentence-1)

[7](#general-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8038)

*Returns*: true if the shared lock was acquired, otherwise false[.](#general-7.sentence-1)

[8](#general-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8042)

*Throws*: Timeout-related exceptions ([[thread.req.timing]](thread.req.timing "32.2.4 Timing specifications"))[.](#general-8.sentence-1)

[9](#general-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8047)

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

[10](#general-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8051)

*Preconditions*: The calling thread has no ownership of the mutex[.](#general-10.sentence-1)

[11](#general-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8055)

*Effects*: The function attempts to obtain shared ownership of the mutex[.](#general-11.sentence-1)

Ifabs_time has already passed, the function attempts to obtain shared
ownership without blocking (as if by calling try_lock_shared())[.](#general-11.sentence-2)

The
function returns before the absolute timeout ([[thread.req.timing]](thread.req.timing "32.2.4 Timing specifications"))
specified by abs_time only if it has obtained shared ownership of the
mutex object[.](#general-11.sentence-3)

[*Note [3](#general-note-3)*:

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[.](#general-11.sentence-4)

â *end note*]

If an exception is thrown then a shared lock has not been acquired for
the current thread[.](#general-11.sentence-5)

[12](#general-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8071)

*Synchronization*: If try_lock_shared_until() returns true, priorunlock() operations on the same object synchronize
with ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races")) this operation[.](#general-12.sentence-1)

[13](#general-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8077)

*Return type*: bool[.](#general-13.sentence-1)

[14](#general-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8080)

*Returns*: true if the shared lock was acquired, otherwise false[.](#general-14.sentence-1)

[15](#general-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8084)

*Throws*: Timeout-related exceptions ([[thread.req.timing]](thread.req.timing "32.2.4 Timing specifications"))[.](#general-15.sentence-1)

#### [32.6.4.5.2](#thread.sharedtimedmutex.class) Class shared_timed_mutex [[thread.sharedtimedmutex.class]](thread.sharedtimedmutex.class)

[ð](#lib:shared_timed_mutex)

namespace std {class shared_timed_mutex {public: shared_timed_mutex(); ~shared_timed_mutex();

 shared_timed_mutex(const shared_timed_mutex&) = delete;
 shared_timed_mutex& operator=(const shared_timed_mutex&) = delete; // exclusive ownershipvoid lock(); // blockingbool 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(); // shared ownershipvoid lock_shared(); // blockingbool try_lock_shared(); template<class Rep, class Period>bool try_lock_shared_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration>bool try_lock_shared_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock_shared(); };}

[1](#thread.sharedtimedmutex.class-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8123)

The class shared_timed_mutex provides a non-recursive mutex with shared
ownership semantics[.](#thread.sharedtimedmutex.class-1.sentence-1)

[2](#thread.sharedtimedmutex.class-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8127)

The class shared_timed_mutex meets
all of the shared timed mutex requirements ([thread.sharedtimedmutex.requirements])[.](#thread.sharedtimedmutex.class-2.sentence-1)

It is a standard-layout class ([[class.prop]](class.prop "11.2 Properties of classes"))[.](#thread.sharedtimedmutex.class-2.sentence-2)

[3](#thread.sharedtimedmutex.class-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8132)

The behavior of a program is undefined if

- [(3.1)](#thread.sharedtimedmutex.class-3.1)

it destroys a shared_timed_mutex object owned by any thread,

- [(3.2)](#thread.sharedtimedmutex.class-3.2)

a thread attempts to recursively gain any ownership of a shared_timed_mutex, or

- [(3.3)](#thread.sharedtimedmutex.class-3.3)

a thread terminates while possessing any ownership of a shared_timed_mutex[.](#thread.sharedtimedmutex.class-3.sentence-1)
