Cross-compiling software with GCCSDK

GCCSDK is a cross-compiling toolkit for RISC OS. Cross-compiling means that we can build software on one platform (eg Linux) that will run on another (RISC OS).

Introduction

Often, there are two tasks in moving a piece of software from one platform to another: building the software, and porting the software. The two are interlinked, but the first step is taking the source files (for instance in C or C++) and compiling them for RISC OS. This may show up problems with lacking functionality which we'll have to implement. Once the program is built, the task of porting can begin. GCCSDK provides functionality so that some command line and some X11, SDL and Qt programs can be built and run unmodified on RISC OS. However other programs may require modification, for example writing a Desktop interface or connecting to printer drivers - this will require additional code.

Why cross-compile? In general, the step of building the software is not very interesting. The software already builds in a Unix environment on (for example) Linux, and we would ideally just use the identical setup to build for RISC OS. Unfortunately, the RISC OS build environment is sufficiently different - filenames, different command line tools, different scripting languages and process model - that it's rare to be able to run an identical build on RISC OS. The programmer then has two choices: either write their own build system specifically for RISC OS, or run the existing build system on Unix and cross-compile.

The latter is usually much less work. Additionally, using Unix as a build system allows the use of fast machines (eg multicore desktops and servers with SSDs) rather than slower RISC OS platforms.

Philosophy

GCCSDK's philosophy is to put the intelligence into the toolkit, rather than implementing the porting changes necessary in the codebase of the program being ported. Often, we get source code of programs to port from upstream projects which are rapidly developing. Once the toolkit knows how to translate particular features to RISC OS, we can easily rebuild the latest version of the upstream software without any further modifications. This substantially reduces the work of keeping ported software up to date and bringing across new features.

Indeed, a separate feature of GCCSDK, the autobuilder, takes this a step further. It will look for source code for a package from the Internet -- for example the Debian repositories. It will download, unpack, apply any local RISC OS patches, and try to build the software. If successful, it will then generate zip archives which can be unpacked on a RISC OS machine, but which are also suitable for download by the PackMan packaging system. Putting a package in the autobuilder allows the latest version to be built with a single step.

Installing GCCSDK

Please see [Using GCCSDK|the GCCSDK installation instructions] for details of how to build the toolkit on a Unix system (Windows and MacOS may work but are less well tested).

Cross-compiling a package

GCCSDK understands a number of standard Unix build systems. We'll consider a number of examples. Once installed, the GCCSDK installation's env directory contains a number of scripts that are frontends to common build tools such as autoconf, make and install.

Minimal shell script build

A few programs have a very simple build system, which is barely more than a shell script -- eg build-me.sh:

#!/bin/sh
gcc -o hello hello.c

ro-path is a script that redirects calls to host tools like gcc to ones that build for RISC OS. For instance:

$ /home/riscos/env/ro-path ./build-me.sh