30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.5 Generic locking algorithms [thread.lock.algorithm]

template<class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);
Requires: Each template parameter type shall satisfy the Cpp17Lockable requirements.
[Note
:
The unique_­lock class template meets these requirements when suitably instantiated.
end note
]
Effects: Calls try_­lock() for each argument in order beginning with the first until all arguments have been processed or a call to try_­lock() fails, either by returning false or by throwing an exception.
If a call to try_­lock() fails, unlock() is called for all prior arguments with no further calls to try_­lock().
Returns: -1 if all calls to try_­lock() returned true, otherwise a zero-based index value that indicates the argument for which try_­lock() returned false.
template<class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);
Requires: Each template parameter type shall satisfy the Cpp17Lockable requirements,
[Note
:
The unique_­lock class template meets these requirements when suitably instantiated.
end note
]
Effects: All arguments are locked via a sequence of calls to lock(), try_­lock(), or unlock() on each argument.
The sequence of calls does not result in deadlock, but is otherwise unspecified.
[Note
:
A deadlock avoidance algorithm such as try-and-back-off must be used, but the specific algorithm is not specified to avoid over-constraining implementations.
end note
]
If a call to lock() or try_­lock() throws an exception, unlock() is called for any argument that had been locked by a call to lock() or try_­lock().