Reader-Writer using semaphores and shared memory in C

Question

I'm trying to make a simple reader/writer program using POSIX named semaphores, its working, but on some systems, it halts immediately on the first semaphore and thats it ... I'm really desperate by now. Can anyone help please? Its working fine on my system, so i can't track the problem by ltrace. (sorry for the comments, I'm from czech republic)

https://www.dropbox.com/s/hfcp44u2r0jd7fy/readerWriter.c

Answer 1

POSIX semaphores are not well suited for application code since they are interruptible. Basically any sort of IO to your processes will mess up your signalling. Please have a look at this post.

So you'd have to be really careful to interpret all error returns from the sem_ functions properly. In the code that you posted there is no such thing.

If your implementation of POSIX supports them, just use rwlocks, they are made for this, are much higher level and don't encounter that difficulty.

Answer 2

In computer science, the readers-writers problems are examples of a common computing problem in concurrency. There are at least three variations of the problems, which deal with situations in which many threads try to access the same shared memory at one time. Some threads may read and some may write, with the constraint that no process may access the share for either reading or writing, while another process is in the act of writing to it. (In particular, it is allowed for two or more readers to access the share at the same time.) A readers-writer lock is a data structure that solves one or more of the readers-writers problems.