33 Concurrency support library [thread]

33.6 Mutual exclusion [thread.mutex]

33.6.4 Mutex requirements [thread.mutex.requirements]

33.6.4.1 In 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.

33.6.4.2 Mutex types [thread.mutex.requirements.mutex]

33.6.4.2.1 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]).

— 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:

(3.1)
resource_unavailable_try_again — if any native handle type manipulated is not available.
(3.2)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(3.3)
invalid_argument — 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.

— 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.

— 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 object synchronize 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:

(12.1)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(12.2)
resource_deadlock_would_occur — 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]).

— 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 subsequent try_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.

— 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.

33.6.4.2.2 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 (for lock()) until the owning thread has released ownership with a call to unlock().

[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.

— 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 calls lock() on that object.

If the implementation can detect the deadlock, a resource_deadlock_would_occur error condition might be observed.

— 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.

33.6.4.2.3 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 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() 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:

(4.1)
it destroys a recursive_mutex object owned by any thread or
(4.2)
a thread terminates while owning a recursive_mutex object.

33.6.4.3 Timed mutex types [thread.timedmutex.requirements]

33.6.4.3.1 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]).

— 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 or shared_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 calling try_lock()).

The function returns within the timeout specified by rel_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]

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 or shared_timed_mutex, the calling thread does not own the mutex.

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 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.

— 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]).

33.6.4.3.2 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(); // 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] }; }

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:

(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.

33.6.4.3.3 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(); // 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] }; }

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).

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 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.

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.

33.6.4.4 Shared mutex types [thread.sharedmutex.requirements]

33.6.4.4.1 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]).

— end note]

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.

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:

(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.

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 subsequent lock() 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.

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] }; }

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:

(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.

shared_mutex may be a synonym for shared_timed_mutex.

33.6.4.5 Shared timed mutex types [thread.sharedtimedmutex.requirements]

33.6.4.5.1 General [thread.sharedtimedmutex.requirements.general]

The standard library type shared_timed_mutex is a shared 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 of duration ([time.duration]), and abs_time denotes an object of an instantiation of time_point .

[Note 1:

The shared timed mutex types meet the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

— 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 calling try_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.

— 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, 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: 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.

If abs_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.

— 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, 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: Timeout-related exceptions ([thread.req.timing]).

33.6.4.5.2 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 ownership 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(); // shared ownership void lock_shared(); // blocking bool 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:

(3.1)
it destroys a shared_timed_mutex object owned by any thread,
(3.2)
a thread attempts to recursively gain any ownership of a shared_timed_mutex, or
(3.3)
a thread terminates while possessing any ownership of a shared_timed_mutex.