53 KiB
[thread.mutex.requirements]
32 Concurrency support library [thread]
32.6 Mutual exclusion [thread.mutex]
32.6.4 Mutex requirements [thread.mutex.requirements]
32.6.4.1 General [thread.mutex.requirements.general]
A mutex object facilitates protection against data races and allows safe synchronization of data between execution agents.
An execution agent owns a mutex from the time it successfully calls one of the lock functions until it calls unlock.
Mutexes can be either recursive or non-recursive, and can grant simultaneous ownership to one or many execution agents.
Both recursive and non-recursive mutexes are supplied.
32.6.4.2 Mutex types [thread.mutex.requirements.mutex]
32.6.4.2.1 General [thread.mutex.requirements.mutex.general]
The mutex types are the standard library types mutex,recursive_mutex, timed_mutex, recursive_timed_mutex,shared_mutex, and shared_timed_mutex.
They meet the requirements set out in [thread.mutex.requirements.mutex].
In this description, m denotes an object of a mutex type.
[Note 1:
The mutex types meet the Cpp17Lockable requirements ([thread.req.lockable.req]).
â end note]
The mutex types meet Cpp17DefaultConstructible and Cpp17Destructible.
If initialization of an object of a mutex type fails, an exception of type system_error is thrown.
The mutex types are neither copyable nor movable.
The error conditions for error codes, if any, reported by member functions of the mutex types are as follows:
-
resource_unavailable_try_again â if any native handle type manipulated is not available.
-
operation_not_permitted â if the thread does not have the privilege to perform the operation.
-
invalid_argument â if any native handle type manipulated as part of mutex construction is incorrect.
The implementation provides lock and unlock operations, as described below.
For purposes of determining the existence of a data race, these behave as atomic operations ([intro.multithread]).
The lock and unlock operations on a single mutex appears to occur in a single total order.
[Note 2:
This can be viewed as the modification order of the mutex.
â end note]
[Note 3:
Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization can be used to ensure that mutex objects are initialized and visible to other threads.
â end note]
The expression m.lock() is well-formed and has the following semantics:
Preconditions: If m is of type mutex, timed_mutex,shared_mutex, or shared_timed_mutex, the calling thread does not own the mutex.
Effects: Blocks the calling thread until ownership of the mutex can be obtained for the calling thread.
Synchronization: Prior unlock() operations on the same objectsynchronize with ([intro.multithread]) this operation.
Postconditions: The calling thread owns the mutex.
Return type: void.
Throws: system_error when an exception is required ([thread.req.exception]).
Error conditions:
-
operation_not_permitted â if the thread does not have the privilege to perform the operation.
-
resource_deadlock_would_occur â if the implementation detects that a deadlock would occur.
The expression m.try_lock() is well-formed and has the following semantics:
Preconditions: If m is of type mutex, timed_mutex,shared_mutex, or shared_timed_mutex, the calling thread does not own the mutex.
Effects: Attempts to obtain ownership of the mutex for the calling thread without blocking.
If ownership is not obtained, there is no effect and try_lock() immediately returns.
An implementation may fail to obtain the lock even if it is not held by any other thread.
[Note 4:
This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange ([atomics]).
â end note]
An implementation should ensure that try_lock() does not consistently return false in the absence of contending mutex acquisitions.
Synchronization: If try_lock() returns true, prior unlock() operations on the same object synchronize with this operation.
[Note 5:
Since lock() does not synchronize with a failed subsequenttry_lock(), the visibility rules are weak enough that little would be known about the state after a failure, even in the absence of spurious failures.
â end note]
Return type: bool.
Returns: true if ownership was obtained, otherwise false.
Throws: Nothing.
The expression m.unlock() is well-formed and has the following semantics:
Preconditions: The calling thread owns the mutex.
Effects: Releases the calling thread's ownership of the mutex.
Return type: void.
Synchronization: This operation synchronizes with subsequent lock operations that obtain ownership on the same object.
Throws: Nothing.
32.6.4.2.2 Class mutex [thread.mutex.class]
namespace std {class mutex {public:constexpr mutex() noexcept; ~mutex();
mutex(const mutex&) = delete; mutex& operator=(const mutex&) = delete; void lock(); bool try_lock(); void unlock(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native]};}
The class mutex provides a non-recursive mutex with exclusive ownership semantics.
If one thread owns a mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (forlock()) until the owning thread has released ownership with a call tounlock().
[Note 1:
After a thread A has called unlock(), releasing a mutex, it is possible for another thread B to lock the same mutex, observe that it is no longer in use, unlock it, and destroy it, before thread A appears to have returned from its unlock call.
Conforming implementations handle such scenarios correctly, as long as thread A does not access the mutex after the unlock call returns.
These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count.
â end note]
The class mutex meets all of the mutex requirements ([thread.mutex.requirements]).
It is a standard-layout class ([class.prop]).
[Note 2:
A program can deadlock if the thread that owns a mutex object callslock() on that object.
If the implementation can detect the deadlock, a resource_deadlock_would_occur error condition might be observed.
â end note]
The behavior of a program is undefined if it destroys a mutex object owned by any thread or a thread terminates while owning a mutex object.
32.6.4.2.3 Class recursive_mutex [thread.mutex.recursive]
namespace std {class recursive_mutex {public: recursive_mutex(); ~recursive_mutex();
recursive_mutex(const recursive_mutex&) = delete; recursive_mutex& operator=(const recursive_mutex&) = delete; void lock(); bool try_lock() noexcept; void unlock(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native]};}
The class recursive_mutex provides a recursive mutex with exclusive ownership semantics.
If one thread owns a recursive_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock()) until the first thread has completely released ownership.
The class recursive_mutex meets all of the mutex requirements ([thread.mutex.requirements]).
It is a standard-layout class ([class.prop]).
A thread that owns a recursive_mutex object may acquire additional levels of ownership by calling lock() or try_lock() 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_mutex object, additional calls to try_lock() fail, and additional calls tolock() throw an exception of type system_error.
A thread shall call unlock() once for each level of ownership acquired by calls tolock() and try_lock().
Only when all levels of ownership have been released may ownership be acquired by another thread.
The behavior of a program is undefined if
it destroys a recursive_mutex object owned by any thread or
a thread terminates while owning a recursive_mutex object.
32.6.4.3 Timed mutex types [thread.timedmutex.requirements]
32.6.4.3.1 General [thread.timedmutex.requirements.general]
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.
[Note 1:
The timed mutex types meet the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
â end note]
The expression m.try_lock_for(rel_time) is well-formed and has the following semantics:
Preconditions: If m is of type timed_mutex orshared_timed_mutex, the calling thread does not own the mutex.
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 callingtry_lock()).
The function returns within the timeout specified byrel_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.
â end note]
Synchronization: If try_lock_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Return type: bool.
Returns: true if ownership was obtained, otherwise false.
Throws: Timeout-related exceptions ([thread.req.timing]).
The expression m.try_lock_until(abs_time) is well-formed and has the following semantics:
Preconditions: If m is of type timed_mutex orshared_timed_mutex, the calling thread does not own the mutex.
Effects: The function attempts to obtain ownership of the mutex.
Ifabs_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 byabs_time only if it has obtained ownership of the mutex object.
[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.
â end note]
Synchronization: If try_lock_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Return type: bool.
Returns: true if ownership was obtained, otherwise false.
Throws: Timeout-related exceptions ([thread.req.timing]).
32.6.4.3.2 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(); // 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(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native]};}
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).
The class timed_mutex meets all of the timed mutex requirements ([thread.timedmutex.requirements]).
It is a standard-layout class ([class.prop]).
The behavior of a program is undefined if
it destroys a timed_mutex object owned by any thread,
a thread that owns a timed_mutex object calls lock(),try_lock(), try_lock_for(), or try_lock_until() on that object, or
a thread terminates while owning a timed_mutex object.
32.6.4.3.3 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(); // blockingbool 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]};}
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 (fortry_lock()) or block (for lock(), try_lock_for(), andtry_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).
The class recursive_timed_mutex meets all of the timed mutex requirements ([thread.timedmutex.requirements]).
It is a standard-layout class ([class.prop]).
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 arecursive_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(), andtry_lock_until().
Only when all levels of ownership have been released may ownership of the object be acquired by another thread.
The behavior of a program is undefined if
it destroys a recursive_timed_mutex object owned by any thread, or
a thread terminates while owning a recursive_timed_mutex object.
32.6.4.4 Shared mutex types [thread.sharedmutex.requirements]
32.6.4.4.1 General [thread.sharedmutex.requirements.general]
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]).
â end note]
In addition to the exclusive lock ownership mode specified in [thread.mutex.requirements.mutex], shared mutex types provide ashared 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:
Preconditions: The calling thread has no ownership of the mutex.
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.
Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Postconditions: The calling thread has a shared lock on the mutex.
Return type: void.
Throws: system_error when an exception is required ([thread.req.exception]).
Error conditions:
-
operation_not_permitted â if the thread does not have the privilege to perform the operation.
-
resource_deadlock_would_occur â if the implementation detects that a deadlock would occur.
The expression m.unlock_shared() is well-formed and has the following semantics:
Preconditions: The calling thread holds a shared lock on the mutex.
Effects: Releases a shared lock on the mutex held by the calling thread.
Return type: void.
Synchronization: This operation synchronizes with subsequentlock() operations that obtain ownership on the same object.
Throws: Nothing.
The expression m.try_lock_shared() is well-formed and has the following semantics:
Preconditions: The calling thread has no ownership of the mutex.
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.
Return type: bool.
Returns: true if the shared lock was acquired, otherwise false.
Throws: Nothing.
32.6.4.4.2 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 ownershipvoid lock(); // blockingbool try_lock(); void unlock(); // shared ownershipvoid lock_shared(); // blockingbool 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
it destroys a shared_mutex object owned by any thread,
a thread attempts to recursively gain any ownership of a shared_mutex, or
a thread terminates while possessing any ownership of a shared_mutex.
shared_mutex may be a synonym for shared_timed_mutex.
32.6.4.5 Shared timed mutex types [thread.sharedtimedmutex.requirements]
32.6.4.5.1 General [thread.sharedtimedmutex.requirements.general]
The standard library type shared_timed_mutex is ashared timed mutex type.
Shared timed mutex types meet the requirements of timed mutex types ([thread.timedmutex.requirements]), shared mutex types ([thread.sharedmutex.requirements]), and additionally meet the requirements set out below.
In this description,m denotes an object of a shared timed mutex type,rel_time denotes an object of an instantiation ofduration ([time.duration]), andabs_time denotes an object of an instantiation oftime_point.
[Note 1:
The shared timed mutex types meet the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).
â end note]
The expression m.try_lock_shared_for(rel_time) is well-formed and has the following semantics:
Preconditions: The calling thread has no ownership of the mutex.
Effects: Attempts to obtain shared lock ownership for the calling thread 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 callingtry_lock_shared()).
The function returns within the timeout specified by rel_time only if it has obtained shared 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.
â end note]
If an exception is thrown then a shared lock has not been acquired for the current thread.
Synchronization: If try_lock_shared_for() returns true, priorunlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Return type: bool.
Returns: true if the shared lock was acquired, otherwise false.
Throws: Timeout-related exceptions ([thread.req.timing]).
The expression m.try_lock_shared_until(abs_time) is well-formed and has the following semantics:
Preconditions: The calling thread has no ownership of the mutex.
Effects: The function attempts to obtain shared ownership of the mutex.
Ifabs_time has already passed, the function attempts to obtain shared ownership without blocking (as if by calling try_lock_shared()).
The function returns before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained shared ownership of the mutex object.
[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.
â end note]
If an exception is thrown then a shared lock has not been acquired for the current thread.
Synchronization: If try_lock_shared_until() returns true, priorunlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Return type: bool.
Returns: true if the shared lock was acquired, otherwise false.
Throws: Timeout-related exceptions ([thread.req.timing]).
32.6.4.5.2 Class shared_timed_mutex [thread.sharedtimedmutex.class]
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(); };}
The class shared_timed_mutex provides a non-recursive mutex with shared ownership semantics.
The class shared_timed_mutex meets all of the shared timed mutex requirements ([thread.sharedtimedmutex.requirements]).
It is a standard-layout class ([class.prop]).
The behavior of a program is undefined if
it destroys a shared_timed_mutex object owned by any thread,
a thread attempts to recursively gain any ownership of a shared_timed_mutex, or
a thread terminates while possessing any ownership of a shared_timed_mutex.