Subj : Re: Boss Told Me You Can't Use Semaphore, But I Have To Do Something
To   : comp.programming.threads
From : Chris Friesen
Date : Wed Jul 13 2005 03:23 pm

David Schwartz wrote:

>     Semaphores were originally designed to be process-shared and do this in 
> a natural way. Mutexes were originally designed to be process-local, and 
> have process-shared support hacked on. For example, to create a semaphore, 
> you just call the semaphore create function. To create a process-shared 
> mutex, you have to allocate shared memory and then put the mutex in it.

With semas, you have something like:

semget() -- creation
semctl() -- initialization
semop() -- lock
semop() -- unlock

With process-shared mutexes you have something like:

mmap() -- creation
pthread_mutex_init() -- initialization
pthread_mutex_lock() -- lock
pthread_mutex_unlock() -- unlock
pthread_mutex_destroy() -- cleanup

There is a 1-to-1 mapping except for the cleanup in the mutex case. 
There's nothing that says a mutex is for threads only, it's just a 
structure in memory.  Whoever has access to that memory can use the mutex.

Also, I actually like the pthreads API much better than the sysV one.

Chris

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