On Jul 13, 2004, at 11:37 AM, Peter Dimov wrote:
Howard Hinnant wrote:
[...]
If the OS provides a mutex that is recursive without documenting that fact, then that sounds like a docs bug to me. If I need a recursive mutex, I will gladly use an OS-supplied one if I can find such a beast. Otherwise I have to build it myself out of an OS-supplied non-recursive mutex. If I find that I've ended up paying for recursion overhead twice, or unnecessarily re-implemented recursive mutexes for that platform, I'll send a bug report to the OS vendor.
However in this case a recursive mutex has been provided by the OS, you just decided to not use it (in order to implement the recursive condition::wait). It seems that you have accepted the responsibility for the tradeoff. For some reason, it also seems that it's time for me to shut up.
Fwiw, imho, you're wrong on both counts. But I'll only address the first: ;-) Here's a little history of the Metrowerks::threads lib. For one, it is fairly new, just released on Mac OS X last year. At that time, I believe 10.2.x was current (can't remember for sure) and 10.2.x did define PTHREAD_MUTEX_RECURSIVE, and I took advantage of that in Metrowerks::recursive_mutex. I also realized at the time that there might be some platforms on which I needed to support where PTHREAD_MUTEX_RECURSIVE wouldn't be defined, and I punted on that (I did not implement a recursive mutex, I downshifted to single thread mode). This was a temporary solution, and I knew I would have to do a better job before I really needed to ship on such a platform. It turns out that I was wrong. Unknown to me was that 10.1.x did not support PTHREAD_MUTEX_RECURSIVE. As I begin to receive these bug reports, I implemented a recursive_mutex based on pthreads, and exposed that implementation only when PTHREAD_MUTEX_RECURSIVE isn't defined. We have not yet shipped this fix, and I'm not the right person to ask for scheduling (I have no idea when it will ship). It is my intention (whether I actually succeed or not is another matter) that I use native recursive mutexes to implement Metrowerks::recursive_mutex when they are provided. Otherwise I implement Metrowerks::recursive_mutex using native non-recursive mutexes. With regards to native recursive mutexes, and their interaction with condition::wait: This is a situation that has come to my attention only with this thread, and I am grateful for that. This is one of the reasons I participate in boost. It makes a better MSL (that's what we call our library). I can report that for those platforms where native recursive mutexes are not available, and thus I have emulated them, the interaction with condition::wait works. I hope that clarifies things. I feel I have not clearly communicated in this thread and hope this post helps to rectify that. -Howard