The Hacking Way: Part 1 - First Steps

Author

Roman Kargin
Copyright (c) 2012 Roman Kargin
Used by permission.

Introduction

Back in the past, I wanted to make the smallest possible executables on UNIX-ish operating systems (SunOS, Tru64, OS9, OpenVMS and others). As a result of my research I wrote a couple of small tutorials for various hacking-related magazines (like Phrack or x25zine). Doing the same on AmigaOS naturally became a topic of interest for me - even more so when I started seeing, in Amiga forums, questions like "Why are AmigaOS4 binaries bigger than they should be?" Therefore I believe that producing small OS4 executables could make an interesting topic for an article. Further in the text I'll explain how ldscripts can help the linker make non-aligned binaries, and cover various other aspects associated with the topic. I hope that at least for programmers the article will be an interesting and thought-provoking read.

Before you go on, please note that it is assumed here that you have basic programming skills and understanding of C and assembler, that you are familiar with BSD syntax, know how UNIX and AmigaOS3/4 work, and that you have the PPC V.4-ABI and ELF specification at hand. But if you don't, there's no need to stop reading as I'll try to cover the basics where necessary.

The Basics

To begin with, let's present and discuss some basic terms and concepts. We'll also dispel some popular myths.

The C standard library (libc)

Thirty years ago, when the C language developed so much that its different implementations started to pose a practical problem, the American National Institute of Standards (ANSI) formed a committee for the standardization of the language. The standard, generally referred to as ANSI C, was finally adopted in 1989 (this is why it is sometimes called C89). Part of this standard was a library including common functions, called the "C standard library", or "C library", or "libc". The library has been an inherent part of all subsequently adopted C standards.

Libc is platform-independent in the sense that it provides the same functionality regardless of operating system - be it UNIX, Linux, AmigaOS, OpenVMS, AROS, whatever. The actual implementation may vary from OS to OS. For example in UNIX, the most popular implementation of the C standard library is glibc (GNU Library C). But there are others: uClibc (for embedded Linux systems, without MMU), dietlibc (as the name suggests, it is meant to compile/link programs to the smallest possible size) or Newlib. Originally developed for a wide range of embedded systems, Newlib is the preferred C standard library in AmigaOS4 and is now part of the kernel.

On AmigaOS4, three implementations of libc are used: clib2, newlib and vclib. The GCC compiler supports clib2 and newlib, the VBCC compiler supports newlib and vclib.

clib2

This is an Amiga-specific implementation originally written from scratch by Olaf Barthel, with some ideas borrowed from the BSD libc implementation, libnix, etc. Under AmigaOS4, clib2 is becoming phased out. The GCC compiler distributed as part of the OS4 SDK uses Newlib by default (as if you used the -mcrt=newlib switch). An important note: clib2 is only available for static linking, while Newlib is opened at runtime (thus making your executables smaller). Clib2 is open source, the latest version can be found here: http://sourceforge.net/projects/clib2/

Newlib

A better and more modern libc implementation. While the AmigaOS4 version is closed source (all adaptations and additional work is done by the OS development team), it's based on the open source version of Newlib. The original version is maintained by RedHat developer Jeff Johnston, and is used in most commercial and non-commercical GCC ports for non-Linux embedded systems: http://www.sourceware.org/newlib/

Newlib does not cover the ANSI C99 standard only: it's an expanded library that also includes common POSIX functions (clib2 implements them as well). But certain POSIX functions - such as glob(), globfree(), or fork() - are missing; and while some of them are easy to implement, others are not - fork() being an example of the latter.

Newlib is also available as a shared object.

vclib

This library was made for the vbcc compiler. Like clib2 it is linked statically, but only provides ANSI C/C99 functions (i.e. no POSIX).