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RE: [lwip-users] use of static variables in tcp_in.c

From: Goldschmidt Simon
Subject: RE: [lwip-users] use of static variables in tcp_in.c
Date: Thu, 4 Oct 2007 10:55:18 +0200

> >  > tcp_in.c in lwIP 1.2.0 makes use of static global 
> variables to pass
> > > arguments between functions. This implies that tcp_input is not
> > re-entrant.
> > > However this doesn't seem to be documented in ethernetif.c or 
> > > anywhere
> > else.
> > > Is there something I'm missing here? 
> > 
> > Does it have to be documented in ethernetif.c? Why would you call 
> > tcp_input more than once? That would mean you would call 
> tcp_input (or 
> > maybe
> > netif->input)
> > from your netif->output function (which may be called in tcp_input).
> > That is
> > not allowed in RAW mode (API mode puts that packet on a 
> queue only, so 
> > the function is not really called again). This is documented, for 
> > example in loopif.c. Maybe it should be documented better...
> That said, is there any compelling reason to use static 
> global variables rather than function arguments?

Hehe, I remember complaining about that over a year ago. At that time,
I think the response was 'leave it as it is'...

Removing any global variables is also a necessary step when giving
the stack more multithreading capabilities (e.g. configuring high-
and low-prio connections to run in different threads...).

I think it would be a good thing moving from global variables to
but I remember adam writing about intentionally using global variables
as a design goal in embedded systems (using a smaller stack, favouring
statically configured memory) which is an opinion I don't share, but we
have to discuss that!


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