creating C-library stubs in raspberry pie bare metal programming

Question

I am currently working with raspberry pie and following a well written chapter 2 tutorial on Chap 2 of valvers.com. I managed to understand everything but I am little stuck on the last section titled "C-Library stubs". In that I am facing some doubts which i mentioned below:

    #include <sys/stat.h>

/* A helper function written in assembler to aid us in allocating memory */
extern caddr_t _get_stack_pointer(void);

/* Increase program data space. As malloc and related functions depend on this,
   it is useful to have a working implementation. The following suffices for a
   standalone system; it exploits the symbol _end automatically defined by the
   GNU linker. */
caddr_t _sbrk( int incr )
{
    extern char _end;
    static char* heap_end;
    char* prev_heap_end;

    if( heap_end == 0 )
        heap_end = &_end;

     prev_heap_end = heap_end;

     if( ( heap_end + incr) > _get_stack_pointer() )
     {
        while(1)
        {
            /* TRAP HERE! */
        }
     }

     heap_end += incr;
     return (caddr_t)prev_heap_end;
}

so far i have only understood that this is the implementation of _sbrk function.

1). I am not able to understand what exactly this code does in reference to the tutorial given in link?

2). The other thing that i am not able to understand is common in armc-008c and 009c i.e. why and how we are changing from main function to kernel_main function? the only text available for this mentions that

NOTE: We've now changed from main to kernel_main, and there's a reason for this - the bootloader is actually expecting a slightly different entry definition compared to the standard C main function. So as we're setting up our own C-Runtime anyway, we can define the correct entry format

I am a beginner in C and these may sound noobish questions but I have did lots of research and spent 3 to 4 days on same tutorial before asking this question here. I am still trying to understand logic behind these programs. Can you guys shed some light over this issue. It will be much more than just help.

thanks in advance

Specification of your question could help increasing the change of a good answer. — moffeltje
Specification as in you want some more details about my question? or you want me to reduce the details i have given? — Dhruvify
I have edited my question and reduced some details. Does it looks clean now? thanks for your feedback. — Dhruvify
I took a fast look to the tutorial link, it seems low level stuff, good to learn and know about but not really used on every day C programming, if you are a C beginner may be better you look for some easier tutorial first. — Alex

Alex Alex · Accepted Answer · 2015-03-17T11:22:00

The _sbrk function in your question is trying to increase the extern symbol _end allowing the application to malloc more memory. (It doesn't directly increment _end but return the incremented value, heap_end declared as static preserve the value through multiple _sbrk calls)

~~(Personally I've some doubt this _sbrk implementation have ever been tested, _end declared as char not pointer (* is missing) is suspect and heap_end assumed to be initialized to 0)~~

About main and kernel_main, almost every C code need an entry point, for common libc application is main (is a libc convention), but as the quote say in this case a bootloader is used and it expect (looks for) a kernel_main symbol to jump into (a bootloader usually doesn't use the libc). The real entry point for (at least gcc) compilers is _start but libc hide it and use it to prepare the environment to launch the application, may be this help you understand how it work.

creating C-library stubs in raspberry pie bare metal programming

1 Answers