Reader-writer lock with condition variable

Question

I find neither boost nor tbb library's condition variable has the interface of working with reader-writer lock (ie. shared mutex in boost). condition_variable::wait() only accepts mutex lock. But I think it's quite reasonable to have it work with reader-writer lock. Can anyone tell me the reason why they don't support that, or why people don't do that?

Thanks, Cui

Answer 1

The underlying platform's native threading API might not be able to support it easily. For example, on a POSIX platform where a condition variable is implemented in terms of pthread_cond_t it can only be used with pthread_mutex_t. In order to get maximum performance the basic condition variable type is a lightweight wrapper over the native types, with no additional overhead.

If you want to use other types of mutex you should use std::condition_variable_any or boost::condition_variable_any, which work with any type of mutex. This has a small additional overhead due to using an internal mutex of the native plaform's type in addition to the user-supplied mutex. (I don't know if TBB offers an equivalent type.)

It's a design trade-off that allows either performance or flexibility. If you want maximum performance you get it with condition_variable but can only use simple mutexes. If you want more flexibility you can get that with condition_variable_any but you must sacrifice a little performance.