Re: [lwip-users] SemSignal gets called on a already signaled semaphore

[Jonathan Larmour]

Matthias Weisser wrote:
> Hi
>
> After playing around with my echo server I saw a strange behavior of my
> sys_sem_signal() implementation. I have implemented the semaphores as
> binary mutexes. Now I see that one of the semaphores is created with a
> initial count of 0 then signaled and then signaled again. Can this be
> ignored as I expect or indicates it another bug in my port?

Semaphores and mutexes are not identical. Especially, as in this case, when 
the semaphores are counting semaphores, not binary. But you can abstract it 
using mutexes and condition variables, and a variable to represent the 
semaphore count.

So waiting on a semaphore would mean:
lock mutex
while (count==0)
  wait on condition variable
count--
unlock mutex

And posting a semaphore:
lock mutex
count++
signal condition variable
unlock mutex

Jifl
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