Gottlob Frege <gottlobfrege@gmail.com> writes:
On Sun, Nov 22, 2009 at 9:08 PM, Vicente Botet Escriba <vicente.botet@wanadoo.fr> wrote:
Hi,
I don't know if I have already requested that but ...
Is there any deep reason to don't provide the equivalent of the free functions lock, try_lock
My guesses at the reasons:
Non-member function lock(Lockable1,Lockable2,...) Non-member function lock(begin,end)
Locking a bunch of locks is a recipe for deadlock. Do we lock them in order? Then hold lock1, lock2, waiting to get lock3? Sounds dangerous.
That's why these functions (which are part of the current boost release) don't lock in order, they use a try-and-back-off algorithm to avoid deadlock.
Non-member function try_lock(Lockable1,Lockable2,...) Non-member function try_lock(begin,end)
Sounds even scarier - after trying lock1, do you try lock2? - so you don't get lock1, you do get lock2, other thread gets the opposite == deadlock.
Again, there is no risk of deadlock here. If a thread fails to acquire one of the locks it releases all the locks it has acquired so far.
to unlock multiple lockables?
Non-member function unlock(Lockable1,Lockable2,...) Non-member function unlock(begin,end)
Only other reason would be because they are not common enough, and easy enough to write on your own (using for_each, etc) for when you do need them.
Precisely. You can just call unlock on each item in turn. There are no complications here: you must own the locks to be able to call unlock, and the order doesn't matter.
Oh, and lastly, because in general direct calls to lock/unlock are discouraged (although at times useful/necessary) and scope_lock is preferred.
Sort of. If your locks are scoped to the current block with unique_lock or lock_guard then you don't need to use a block-unlock function, even if you used lock() to acquire them all (having constructed the unique_lock instances with defer_lock). Anthony -- Author of C++ Concurrency in Action | http://www.stdthread.co.uk/book/ just::thread C++0x thread library | http://www.stdthread.co.uk Just Software Solutions Ltd | http://www.justsoftwaresolutions.co.uk 15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976