On 3/19/07, Momchil Velikov <chill@xxxxxxxxx> wrote:
Daniel Rodrick wrote:
> I may be wrong, but from what I understand, the count passed in the
> semaphore initialization
>
> sema_init(sem, count);
>
> is the number of threads that are allowed to hold the semaphore
> simultaneously (no of threads that are allowed to enter their critical
> sections simultaneously). So can't a semaphore be actually held
> recursively?
Semaphores are not "held". See below.
> Also, the only difference I could find between a semaphore and mutes
> is that a mutex is a specail and most common form of semaphore where
> only one thread can get it at a time. Are there any other differences?
The difference is that a semaphore has no owner. Neither semaphores nor
mutexes are a "special form" of the other - they are fundamentally
distinct mechanisms.
But I see mutexes implemented using semaphores only:
#define __DECLARE_SEMAPHORE_GENERIC(name,count) \
struct semaphore name = __SEMAPHORE_INITIALIZER(name,count)
#define DECLARE_MUTEX(name) __DECLARE_SEMAPHORE_GENERIC(name,1)
#define DECLARE_MUTEX_LOCKED(name) __DECLARE_SEMAPHORE_GENERIC(name,0)
??
Dan
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