A mutex object facilitates protection against data races and allows safe synchronization of data between execution agents ([thread.req.lockable]). 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. The mutex types supplied by the standard library provide exclusive ownership semantics: only one thread may own the mutex at a time. Both recursive and non-recursive mutexes are supplied.
The mutex types are the standard library types std::mutex, std::recursive_mutex, std::timed_mutex, and std::recursive_timed_mutex. They shall meet the requirements set out in this section. In this description, m denotes an object of a mutex type.
The mutex types shall meet the Lockable requirements ([thread.req.lockable.req]).
The mutex types shall be DefaultConstructible and Destructible. If initialization of an object of a mutex type fails, an exception of type system_error shall be thrown. The mutex types shall not be copyable or movable.
The error conditions for error codes, if any, reported by member functions of the mutex types shall be:
resource_unavailable_try_again — if any native handle type manipulated is not available.
operation_not_permitted — if the thread does not have the privilege to perform the operation.
device_or_resource_busy — if any native handle type manipulated is already locked.
invalid_argument — if any native handle type manipulated as part of mutex construction is incorrect.
The implementation shall provide 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 shall appear to occur in a single total order. [ Note: this can be viewed as the modification order ([intro.multithread]) of the mutex. — end note ] [ Note: Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization should be used to ensure that mutex objects are initialized and visible to other threads. — end note ]
The expression m.lock() shall be well-formed and have the following semantics:
Requires: If m is of type std::mutex or std::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.
Postcondition: The calling thread owns the mutex.
Return type: void
Synchronization: Prior unlock() operations on the same object shall synchronize with ([intro.multithread]) this operation.
Throws: system_error when an exception is required ([thread.req.exception]).
Error conditions:
operation_not_permitted — if the thread does not have the privilege to perform the operation.
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.
device_or_resource_busy — if the mutex is already locked and blocking is not possible.
The expression m.try_lock() shall be well-formed and have the following semantics:
Requires: If m is of type std::mutex or std::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: This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange (Clause [atomics]). — end note ] An implementation should ensure that try_lock() does not consistently return false in the absence of contending mutex acquisitions.
Return type: bool
Returns: true if ownership of the mutex was obtained for the calling thread, otherwise false.
Synchronization: If try_lock() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation. [ Note: 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 ]
Throws: Nothing.
The expression m.unlock() shall be well-formed and have the following semantics:
Requires: The calling thread shall own the mutex.
Effects: Releases the calling thread's ownership of the mutex.
Return type: void
Synchronization: This operation synchronizes with ([intro.multithread]) subsequent lock operations that obtain ownership on the same object.
Throws: Nothing.
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();
typedef implementation-defined native_handle_type; // 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: 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. Implementations are required to handle such scenarios correctly, as long as thread A doesn't 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 shall satisfy all the Mutex requirements ([thread.mutex.requirements]). It shall be a standard-layout class (Clause [class]).
[ Note: A program may 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 may 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.
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();
typedef implementation-defined native_handle_type; // 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 shall satisfy all the Mutex requirements ([thread.mutex.requirements]). It shall be a standard-layout class (Clause [class]).
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() shall fail, and additional calls to lock() shall 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 timed mutex types are the standard library types std::timed_mutex and std::recursive_timed_mutex. They shall 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 ([time.duration]), and abs_time denotes an object of an instantiation of time_point ([time.point]).
The timed mutex types shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
The expression m.try_lock_for(rel_time) shall be well-formed and have the following semantics:
Requires: If the tick period of rel_time is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. If m is of type std::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 shall return within the timeout specified by rel_time only if it has obtained ownership of the mutex object. [ Note: 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 ]
Return type: bool
Returns: true if ownership was obtained, otherwise false.
Synchronization: If try_lock_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Throws: Nothing.
The expression m.try_lock_until(abs_time) shall be well-formed and have the following semantics:
Requires: If m is of type std::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 shall return before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained ownership of the mutex object. [ Note: 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 ]
Return type: bool
Returns: true if ownership was obtained, otherwise false.
Synchronization: If try_lock_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Throws: Nothing.
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();
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();
typedef implementation-defined native_handle_type; // 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 shall satisfy all of the TimedMutex requirements ([thread.timedmutex.requirements]). It shall be a standard-layout class (Clause [class]).
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();
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();
typedef implementation-defined native_handle_type; // 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 shall satisfy all of the TimedMutex requirements ([thread.timedmutex.requirements]). It shall be a standard-layout class (Clause [class]).
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() shall fail, and additional calls to lock() shall 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.