[Icc-avr] Are 'local' vars in main automatuc or in bss?

[Jaspers, Ton]

Hear, hear!

This is exactly the stuff I use in the selection process of new
colleagues. 
 
Ton
 

> -----Original Message-----
> From: icc-avr-bounces at imagecraft.com 
> [mailto:icc-avr-bounces at imagecraft.com] On Behalf Of David Brown
> Sent: vrijdag 26 oktober 2007 10:28
> To: Discussion list for ICCAVR and ICCtiny Users. You do NOT 
> need to subscribe to icc-announce if you are a member of this.
> Subject: Re: [Icc-avr] Are 'local' vars in main automatuc or in bss?
> 
> Graham Whyte wrote:
> > main() is no different from any other subroutine except 
> that it gets 
> > called at startup
> >  
> > Variables declared outside subroutines or functions in any 
> module file 
> > are Global within that module The can be referenced from anywhere 
> > within that module
> 
> Correct up to here...
> 
> > To make them Global to other modules, each other module needs to 
> > declare then as External
> 
> That's wrong, and very misleading.
> 
> Variables (and functions) get global linkage if they are 
> defined at file scope without "static" - C defaults to making 
> them global and externally accessible.  Thus two "int x;" 
> definitions at file scope in two modules refer to the same 
> variable at the same address, regardless of any "extern" declarations.
> 
> The point of "extern" is to tell the compiler about a global 
> variable (or function) without defining it - it allows access 
> to a global variable, but does not turn a variable into a global one.
> 
> > This is exactly the same as making a subroutine available to other 
> > modules The declarations must of course be identical
> >  
> 
> The declaration *should* be identical - but be careful, 
> because C allows you to get it completely wrong and still be legal C.
> 
> > You can make this very neat and tidy by putting the Globals in a 
> > separate module Then create the external  declarations in a 
> separate 
> > header file which you can #include wherever you need You 
> then only got 
> > one copy to worry about
> 
> I've seen programs made like this - as Ton says, "Yuck!".
> 
> If you want to write programs that are readable, 
> understandable, debugable and maintainable, and allow for at 
> least some level of re-use between programs, then you will 
> avoid this setup like the plague.
> 
> Modularisation is critical to writing software.  Break your 
> program into parts that fit logically together, and figure 
> out what data and functions (and types, macros, etc.) that 
> module makes available to other parts of the whole program.  
> This information is your module's interface, or API, and 
> forms its "module.h" header file.
> 
> The header file "module.h" contains comments about how to use 
> the module, along with all type declarations, #define's, "extern" 
> declarations of data and functions, and any #include's that 
> are needed to *use* the module (for example, if it contains a 
> line "extern uint8_t count;", then it should also contain a 
> line "#include <stdint.h>" or equivalent).
> 
> The C file "module.c" contains the *implementation* of the 
> module, and has #include "module." near the beginning.  Every 
> file-scope variable or function is either private to the 
> module, and defined "static", *or* it has a corresponding 
> "extern" declaration in "module.h".  There should be no 
> exceptions (except perhaps temporarily to aid debugging, 
> since you can see non-static items in the map file).
> 
> 
> As I said in an earlier post, I tend to rant a bit and treat 
> my methods as the only correct methods, but in this case 
> there is no real doubt. 
> If you look at any other properly structured programming 
> language, such as Pascal, Modula 2, or Ada, you'll see that 
> this is exactly the way good modular programming is always 
> done - other languages enforce these rules.
> 
> mvh.,
> 
> David
> 
> 
> >  
> > Graham
> >  
> > -----Original Message-----
> > *From:* icc-avr-bounces at imagecraft.com 
> > [mailto:icc-avr-bounces at imagecraft.com]*On Behalf Of 
> > *bobgardner at aol.com
> > *Sent:* 25 October 2007 14:30
> > *To:* icc-avr at imagecraft.com
> > *Subject:* [Icc-avr] Are 'local' vars in main automatuc or in bss?
> > 
> > Great summary of 'static' vars.... I remember in pascal the scope 
> > 'nested' and stuff 'up a level' was visible, but c doesnt 
> allow nested 
> > functions I guess. Just to clarify, if I declare vars in main, are 
> > they local or in the bss? Is main a subroutine that is called with 
> > params and returns a value? Is the iccavr microcontroller model 
> > different from a ram based system with an os?
> > 
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