Goldschmidt Simon wrote:
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
Jonathan,
As a clarification of the above example, is the intention for the
mutex used by the semaphore pend to be the same as the one
used by the
semaphore post? I imagine not, otherwise the pend would wait on the
condition variable with the mutex locked, therefore
preventing a post
from occurring...
It is the same mutex. Condition variables always have a mutex
associated with them. When you wait on a CV, the mutex is
automatically unlocked by the OS, and when you wake up after
the CV is signalled, the OS automatically relocks the mutex
before the wait function returns.
Reading that explanation I'm afraid the OSes I use don't have such
condition variables! So making them mandatory for lwIP seems not
such a portable idea to me...
Really? I'm amazed if so. Any OSes I should have heard of, or homegrown? I
am sort of assuming that if someone can write a homegrown OS they can
implement mutexes/condition variables for it if they need to.
In any case, I think you can implement mutexes/condition variables in
terms of semaphores if you also have a unique way to identify a thread.
Obviously you wouldn't be able to implement any method of preventing
priority inversion, but that's no worse than at present.
Jifl
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