Re: HELP with linker script!!!

[Vincent Rubiolo <vincent dot rubiolo at windriver dot com>]

Hello Pieter,

Pieter Arnout wrote:
[...]
> (1) How do I indicate the start and end of the cacheable .bss or 
> uncacheable .bss in your example? Don't they usually have symbol names 
> that you can use to refer to them by?
I mentioned possible symbol names for the start of these sections. 
However, these usually do not have any end that is predefined. The 
linker script defines the start symbols of these sections for the 
initialization code to position to corresponding pointers.

> In Grigory's example he writes in C code "extern int my_section_start".
> Is this the way to define such a thing? Seems to me that the linker 
> script couldn't possibly know that the intention of this value is to 
> serve as a boundary for the section my_section ...
No, Grigory's code is a way to reference it. You generally would only 
define the symbol yourself in the linker script. Here is what you can 
toss out :
__bss_end = . ;
This would define the __bss_end symbol and assign the location counter 
value to it. By referencing it as 'extern' in your C code, you can use 
it in your program/libraries.
You can have more information about linker scripts by passing the 
--verbose flag to your ld. It will display the default linker script it 
uses.

> (2) You say:
>
> > Normally the stack and heap do not have a size.  Instead they occupy 
> > all of the memory available to the application which has not already 
> > been allocated to the loaded code and its data.
>
> This makes no sense to me. Frequently the stack and heap grow towards 
> each other, so isn't it important to define a boundary which they cannot 
> cross? How can they not have a size then? Do they just grow towards each 
> other until one collides with the other?
Indeed. The 'boundary check', would you want to have one, should be done 
by your memory/program routines. This is not something that the linker 
is meant to do (the linker only deals with symbols and program 
sections/segments placement, not with program dynamic execution).
> (3) Finally I would love to find a book that was as helpful in 
> explaining this as your email, so I don't have to keep asking these 
> questions. Do you know of a good book that covers these issues?
The binutils manual (http://sources.redhat.com/binutils/docs-2.15/) is 
of course the reference for the tools. However, it is a bit scarce on 
theory. You can consult John R. Levine's book which is, to my knowledge, 
the only book to deal with the subject (could be twice as thick 
however). You can find its only version here 
(http://www.iecc.com/linker/) or but it through your favorite book 
retail (I got mine via Amazon). A very good buy IMHO.

HTH,

Vincent
[...]

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