Question

I am currently building a thin C++ wrapper around pthreads for in-house use. Windows as well as QNX are targeted and fortunately the pthreads-win32 ports seems to work very well, while QNX is conformant to POSIX for our practical purposes.

Now, while implementing semaphores, I hit the function

sem_post_multiple(sem_t*, int)

which is apparently only available on pthreads-win32 but is missing from QNX. As the name suggests the function is supposed to increment the semaphore by the count given as second argument. As far as I can tell the function is not part of neither POSIX 1b nor POSIX 1c.

Although there is currently no requirement for said function I am still wondering why pthreads-win32 provides the function and whether it might be of use. I could try to mimic it for QNX using similar to the following:

sem_post_multiple_qnx(sem_t* sem, int count)
{
   for(;count > 0; --count)
   {
       sem_post(sem);
   }
} 

What I am asking for are suggestion/advice on how to proceed. If consensus suggests to do implement the function for QNX I would also appreciate comments on whether the suggested code snipped is a viable solution.

Thanks in advance.

PS: I deliberately left out my fancy C++ class for clarity. For all folks suggesting boost to the rescue: it is not an option in my current project due to management reasons.

Answer 1

In any case semaphores are an optional extension in POSIX. E.g OS X doesn’t seem to implement them fully. So if you are concerned with portability, you’d have to provide wrappers of the functionalities you need, anyhow.

Your approach to emulate an atomic increment by iterated sem_post has certainly downsides.

• It might be performing badly,
  where usually sem_t are used in
  performance critical contexts.
• This operation would not be
  atomic. So confusing things might
  happen before you finish the loop.

I would stick to the just necessary, strictly POSIX conforming. Beware that sem_timedwait is yet another optional part of the semaphore option.