Re: CRT0 questions

[Jeff Johnston <jjohnstn at redhat dot com>]

Chien-Lung Wu wrote:
> Hi, everyone: 
> I created cross-compiler for our embedded project, using Powerpc (MPC8250).
> It is fine to compile all my application files using cross-compiler with
> newlib C-library. 
> But I got linking stage error when link all object files. I have several
> questions about newlib. Can anyone help me out?

Before you go any further, I suggest you look in libgloss/rs6000.  You will find 
a number of sets of ld scripts for the powerpc.  When you compile/link, you need 
to specify the -T option with a .ld script.  For example, powerpc-elf-gcc 
-Tmbx.ld my.c ....

There is already a crt0.S file there which may suit your needs.  You might need 
to  tweak one of the ld scripts to match your board.  I would start by trying 
the existing ones first.

> Q. crt0 
> When I generate newlib C library, I believe that the crt0 also been
> generated. Since I used company-designed hardware, I need to re-write crt0. 
> Are there information to write crt0? Anything I got to take care when I
> write the crt0? 

There is some information in the libgloss docs (make info in the libgloss 
directory or just look at the sourcefile: libgloss/doc/porting.texi).

> 1. what does crt0? 
> 2. How many things got to do (init) within crt0?

The doc should tell you what you need to know.  Basically, you need to 
initialize the state so that calling main et al works.  Usually, this minimally 
entails setting up the stack pointer and clearing the bss area, however, you 
might need to other things as well (for example, you might have to initialize a 
frame pointer register).  It all depends on the platform.  I would imagine the 
crt0.S in the rs6000 directory should be adequate or very close to what you need.

> 3. After I finish "initialize", what is the "hook" to connect with
> C-library? What function I got to 
> call so that the program can connect with "main()"? for example, I used
> diab4.4b compiler before. 
> I know that the crt0 finally call __init_main to connect with program's main
> function. What is the exact function for newlib C-library?)

newlib doesn't have a main hook.  In some cases, you may want to call a __main 
to do extra stuff like running the C++ constructors but this would also be in 
libgloss.  If you are just using C, you don't need __main and can call main() 
directly.  Once main() returns you need to call exit().  You can see this when 
you look at the libgloss/rs6000/crt0.S file.

> Thanks for your help. 
> Chien-Lung