Cross-compiling for the Raspberry Pi
Introduction
Creating a cross-compiler is a fairly advanced process but can be useful for heavy use. This guide will give some details on how to create a cross-compiler for use with openFrameworks 0.9.0. For older releases (OF 0.8.4/Wheezy) see Raspberry Pi Cross-compiling Guide for Wheezy
Do you need it?
The RPI2/arm7 is much faster and can natively compile openFrameworks in about 15 minutes by utilizing all 4 cores (make -j4). Applications are typically much faster depending on complexity, use of ofxAddons, etc.
Compiling openFrameworks on the RPI1/arm6 is much slower and can take over an hour. Applications can be much faster however still slow.
A typical cross-compiler on a modern Desktop PC will compile the OF core library in about 3 minutes.
Cross-compiling process
To use a cross-compiler you will need to setup a few things:
Toolchain
This is basically a custom version of GCC that understands your target platform. In order to cross-compile openFrameworks the compiler must support GCC 4.9. Previous versions of GCC do not support the level of C++11 features that openFrameworks requires. At the time of openFrameworks 0.9.0 release, the offical Raspberry Pi tools were out-of-date, requiring you to build GCC yourself. The tools were updated with GCC 4.9.3 on February 14, 2016 but have yet to be verified.
Target Root File System
This is a copy of the RPI's file system that the compiler will use to build openFrameworks and applications. It is a copy because you will need to manipulate certain files that if changed directly would cause the RPI to stop working.
Makefile variables
The openFrameworks Makefile system has some built-in variables that allow for easier cross-compiling.
TOOLCHAIN_ROOT
TOOLCHAIN_ROOT is a path to the directory that contains your Toolchain. If you don't pass this in OF will guess a default value of /opt/cross/bin
GCC_PREFIX
A toolchain is often set up with multiple architectures in order to cross-compile a project for multiple platforms. GCC_PREFIX is used to differentiate between these platforms.
For example, to compile for an RPI/Arch Linux target the prefix is often armv6l-unknown-linux-gnueabihf. For Raspbian it is commonly arm-linux-gnueabihf
If GCC_PREFIX is not passed in arm-linux-gnueabihf is assumed. The prefix is then added to -g++, -gcc, -ar and -ld to complete the paths to the compiler and linkers.
RPI_ROOT
RPI_ROOT is the Target Root File System previously described. This is required to be passed in and there is no default value.
These variables are either passed in with the make command:
make TOOLCHAIN_ROOT=/path/to/toolchain GCC_PREFIX=arm-linux-gnueabihf RPI_ROOT=/path/to/local_copy_of_pi_filesystem
or set via environmental variables
export TOOLCHAIN_ROOT=/path/to/toolchain
export GCC_PREFIX=arm-linux-gnueabihf
export RPI_ROOT=/path/to/local_copy_of_pi_filesystem
Using export allows you just use the shorter make command throughout a session. You can make these variables permanent by adding the export lines to the end of ~/.profile.