Guidelines for cross-build friendly packages

This page describes good (and bad) practice in packaging (and upstream code) with respect to cross-building Debian-based packages. Little of this is yet mnandated by policy, but some is and more will be in the future.

There are often lots of ways of 'fixing' a cross-build problem, but unless you are expert it he area it will not be clear what the implications of using one method over another are. This page endeavours to document standard guidelines so that packagers can use it as a reference, and Debian and derivatives can develop policy in this area.

Terminology

This somewhat confusing terminology is GNU's fault.

Principles

dpkg-architecture is responsible for translating between triplets, Debian architecture names and multiarch paths. This provides a consistent interface and namespace.

cross-building is defined as BUILD != HOST. more explicitly in debian.rules: ifeq ($(DEB_BUILD_GNU_TYPE),$(DEB_HOST_GNU_TYPE))

else

endif

We cannot run tests to determine aspects of the host machine which will be different on the build machine.

Classic cross-compiling assumes never running any host architecture binaries, and this remains good policy, but we often have QEMU or similar emulation available these days so it makes sense to control this separately where we can. In practice this means running tests, which should be controlled with DEB_BUILD_OPTS=nocheck rather than 'are we cross-compiling or not'. Cross-build tools will set this by default, but it can be enabled when approapriate.

When cross-building the build must select the correct tools who's output varies with architecture. This is usually done with an explicit GNU triplet prefix (arm-linux-gnueabi- ia64-linux-gnu-). This applies to the following tools: gcc, g++, binutils (ld, as, strip, ar, readelf etc), pkg-config

Build environment

Various things need to be set in the environment for cross-builds to work correctly. These are the things that are set by various tools:

dpkg-buildpackage

all the dpkg-architecture variables:

DEB_BUILD_ARCH
DEB_BUILD_ARCH_OS
DEB_BUILD_ARCH_CPU
DEB_BUILD_ARCH_BITS
DEB_BUILD_ARCH_ENDIAN
DEB_BUILD_GNU_CPU
DEB_BUILD_GNU_SYSTEM
DEB_BUILD_GNU_TYPE
DEB_BUILD_MULTIARCH
DEB_HOST_ARCH
DEB_HOST_ARCH_OS
DEB_HOST_ARCH_CPU
DEB_HOST_ARCH_BITS
DEB_HOST_ARCH_ENDIAN
DEB_HOST_GNU_CPU
DEB_HOST_GNU_SYSTEM
DEB_HOST_GNU_TYPE
DEB_HOST_MULTIARCH

(it also sets some FAKEROOT stuff that we don't care about here, and sanitises a load of stuff out of your normal environment)

sbuild

sbuild duses dpkg-buildpackage so sets the same things as that, and:

CONFIG_SITE/etc/dpkg-cross/cross-config.$DEB_HOST_ARCH  (for autoconf cache settings provided by dpkg-cross)
DEB_BUILD_OPTS+=nocheck

anything else configured to be set in build environment

xdeb

xdeb duses dpkg-buildpackage so sets the same things as that, and:

CONFIG_SITE=/etc/dpkg-cross/cross-config.$DEB_HOST_ARCH  (for autoconf cache settings provided by dpkg-cross)
DEB_BUILD_OPTS+=nocheck
GTEST_INCLUDEDIR=/usr/$DEB_HOST_GNU_TYPE/include
GTEST_LIBDIR=/usr/$DEB_HOST_GNU_TYPE/lib

make

Make sets some things itself too. Some of them not very helpfully, like the implicit $(CC)=cc

Setting correct compiler

Simply hardcoding 'gcc' will not allow cross-building, and is arguably wrong for other reasons.

Normally you want to set gcc for native builds and $(DEB_HOST_GNU_TYPE)-gcc for cross-builds. Sadly we can't just set $(DEB_HOST_GNU_TYPE)-gcc always because that doesn't work natively

So normally people do this:

ifeq ($(DEB_BUILD_GNU_TYPE),$(DEB_HOST_GNU_TYPE))
       CC=gcc
else
       CC=$(DEB_HOST_GNU_TYPE)-gcc
endif

But it should also be possible to override the compiler for the build from the environment so that for example LLVM test builds or strangly-named external toolchains can be used.

Unfortunately make always sets $(CC) (to 'cc') so you can't just test if it is already set, and set it if not. CC ?= $(CC)

This state can be detected with $(origin CC) see http://theory.uwinnipeg.ca/localfiles/infofiles/make/make_80.html I wonder if we should be doing something clever with ifeq "$(origin CC)" "default" ?...

Running/skipping tests

Normally you don't want to run tests when cross-building.