Subj : Re: Concurrent reads and sequential writes
To   : comp.programming.threads
From : Torsten Robitzki
Date : Tue Jul 26 2005 06:54 pm

kozlovsky@kozlovsky-c4.sipo.gess.ethz.ch wrote:

> Hello,
> 
> I am writing a spider application on Linux, C++, STL, pthreads.
> The application has a cache that contains (word, handle) pairs.
> The Cache class has method "get", that, given a word, returns its
> handle (if found in the cache), and method "add", that adds a
> new pair to the cache. About 95% of cache access are get's.
> 
> At present, the cache is synchronized with a single mutex, so
> only a single thread can access the cache at any given time.
> I would like to change synchronization in such a way that any
> number of threads can call "get" at the same time, while "add"
> must remain exclusive, i.e. only one thread can execute "add"
> after blocking threads from starting new "get" and waiting
> till threads currently executing "get" are all finished.
> 
> I cannot figure out how to do it with mutexes, although this
> seems to be a quite common situation, is it? Should I use
> semaphores? Or may be sig_atomic_t? Why should I try to
> reinvent the bycicle? Perhaps there is a known simple
> solution to my problem. Please, help.

I think You are looking for a read/write mutex. Have a look at 
pthread_rwlock_init() and friends. Lock the mutex for reading in the 
get() function and lock the mutex for writing in the add() function.

regards
Torsten

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