Bare Metal C - libraries

Introduction

In my last bare metal C post I was able to complete a hello world program so that our RPI1B can do terminal input /output.  It was quite an achievement to dispense with our operating system and establish communications with the program running on hardware.

Now I could carry on in that direction and write everything else I need in C, it would include memory management, device drivers, utility functions and all the other things that C programmers can usually take for granted.  Life is too short for that so I recognise what I really need is a library, in particular the C-Library which provides so many basic necessities of C life.

However the C library makes operating system calls to the linux (or other) kernel whenever it need assistance in completing functions and I decided I didn't want an Operating System!  I didn't want the large amounts of useful and useless code that comes with it.  So our mission is to provide, ie write, the code needed by the C library.

C runtime

We also need to initialise the C environment which is the function of C-runtime (crt0.o). This is fairly easy and we deal with  it first. Brian Sidebothan provide an excellent description in part 2 of his Valvers Bare Metal C tutorial.  At its simplest we just have to setup a stack pointer so that C can use a stack.  This is set to the program load address 0x8000 on RPI1B.  The program is loaded in addresses from 0x8000 upwards and the stack uses memory downwards towards 0x0000. A second job that is required is to initialise variables to 0, which is a C standard requirement.  The variables are stored in the BSS segment and a small C program (cstartup) can be written to set values in the segment.

Library stubs

The tutorial goes on to explain how we can start using library functions and uses malloc (memory allocation) function to demonstrate.  Compiling a C program containing malloc functions gives an error "_sbrk" not found.  This is a low level function which we need to provide ourselves.  Luckily we can use a working example from newlib.  We copy this to a file c-stubs.c, compile without errors, and we can request / use memory allocated to our program.  In fact newlib contains a list of system calls which the C library expects.  Many of these are just dummy stubs with the function call and no code so you still have to do some work yourself.

Test Program

To demonstrate that I now have a working C library capability I use malloc and strcpy (string copy) in my "Hello World" program.  The program is based on David Welch's UART tutorial example with Valvers additions to use libraries.  The linker script and bss initialisation was a little difficult to write but David Welch comes to the rescue with his explanation of linking requirements.  RPI has simplified linking requirements as all code and data is incorporated into kernel.img and loaded into memory.  Initialising BSSS is not required - although I tested the program successfully both with and without initialisation.