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RE: [lwip-users] About global parameter initial problem

From: bill
Subject: RE: [lwip-users] About global parameter initial problem
Date: Mon, 1 Dec 2008 09:56:04 -0500

I know I'm a bit late adding to this thread.  Good post David.

One point just to be sure it's understood is that your use of the term
static below doesn't mean variables declared with the static qualifier, but
any non-local variable - both globals and those declared static.

One point on Jim's comment: "unless you have a very weird target that does
not use zero as a NULL pointer".  I seem to recall the ANSI/ISO C Standard
includes a requirement that NULL pointers are 0 in arithmetic value.  It's
not handy to look up but I will check tonight.

There is a lot of existing code written with 'if (ptr) ...' instead of 'if
(ptr != NULL) ... which makes the requirement justified.


> Section 6.5.7 Initialization:
> "... If an object that has automatic storage duration is not
> initialized
> explicitly, its value is indeterminate. If an object that has static
> storage
> duration is not initialized explictly, it is initialized implicitly as
> if
> every member that has arithmetic type were assigned 0 and every member
> that
> has pointer type were assigned a null pointer constant. ..."
> Further comments from me:
> Variables declared outside functions are by definition static. The
> above
> rules require that all static variables be set to their initial values
> before the first C function in the application is called, and the
> initial
> value is defined to be zero for all static variables which don't have
> an
> explicit initializer.
> Compiler-supplied startup code must clear all "uninitialized" static
> variables, or the compiler is not compliant with the ANSI C standard.
> User-supplied startup code must do the same, unless the user is willing
> to
> violate the ANSI C standard in this area and deal with the consequences
> in
> any code they didn't write themselves.
> I think it is perfectly reasonable for LWIP to expect ANSI C compliance
> in
> this area, and the standard distribution of LWIP should not have
> explicit
> assignment to zero on static variables.
> A good reason for not doing it this way: in an embedded system, the
> initial
> values for initialized variables are typically loaded into a section in
> and copied to RAM by the startup code. Many embedded compilers will
> ignore
> the fact that the initializer is zero, resulting in a lot more ROM
> space
> being taken up by initialized variable values. Explicitly initializing
> variables to zero will therefore waste ROM space, which is often at a
> premium in smaller systems.
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