Re: [boost] [thread] RFC standard proposed mutex, read-write mutex, condition

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-08-25 21:10:01

I've thought a bit more about the various options that a vendor has with 
regards to sizeof mutex, condition, and shared_mutex (*), and this has 
indirectly made me realize why I feel uncomfortable with the native_handle 
accessor.

It can't be used in portable code. At the same time, its presence puts 
pressure on you as an implementor to make std::mutex be a pthread_mutex_t 
(or a pointer to a pthread_mutex_t) since the OS is POSIX-compliant. This 
removes some of your implementation options; even if you don't find the 
underlying pthread_mutex_t particularly lean or well-performing, you'll be 
reluctant to replace it with an alternative.

On Windows, some vendors will choose HANDLE and some - CRITICAL_SECTION* as 
the native handle, so it's not portable even across implementations on the 
same platform.

In N2178, I've proposed pthread_mutex_t as part of the package, so while a 
"native handle" accessor would have the same disadvantage of tying 
std::mutex to pthread_mutex_t, it would've had the advantage of being 
portable.

(*)

mutex: 4, 8, 16, 44
condition: 4, 28, 32
shared_mutex: 4, 8, sizeof(pthread_rwlock_t), 104

are some of the options that come to mind, using the previously posted sizes 
as an arbitrary baseline.