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 mandated 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 an expert in the area it will not be clear what the implications of using one method over another are. This page endeavors 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:

   1 ifeq ($(DEB_BUILD_GNU_TYPE),$(DEB_HOST_GNU_TYPE))
   2   native build
   3 else
   4   crossbuild
   5 endif
   6 

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

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.

Salsa CI

The default Salsa-CI pipeline now includes a test-crossbuild-arm64 job, that will attempt to cross-compile your package on the default amd64 build architecture for the arm64 host architecture.

So use this default pipeline to get automatic feedback whether your package is cross-compilable (or not).

NOTE: Currently the test-crossbuild-arm64 job has the allow_failure: true property set, so even if the cross-build job fails, the entire pipeline will still be reported as success. You can either manually check the status of each job of the pipeline, or override the property to false in your pipeline definition.

Build environment

Various things need to be set in the environment for cross-builds to work correctly.

Below is a list of what is set by various tools, however the interface to package building is still defined as the debian/rules targets and your rules file should not rely on any outside environment setting.

The recommended way to set the architecture variables provided by dpkg-architecture is to include this snippet provided by dpkg-dev:

   1 include /usr/share/dpkg/architecture.mk
   2 

This is available from dpkg-dev 1.16.1 onward, and will not overwrite any previously-supplied values.

Environment set by build 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 sanitizes a load of stuff out of your normal environment.)

sbuild

sbuild uses 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.

pbuilder

pbuilder automatically adds nocheck to DEB_BUILD_OPTIONS and DEB_BUILD_PROFILES unless passing --no-auto-cross.

xdeb

xdeb uses 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.

Build-Dependencies

Many Build-Dependencies must use the architecture of the HOST architecture (e.g. C-libraries you want to link against).

Other Build-Dependencies must be executed on the BUILD machine. These are obviously compilers, but also interpreters, documentation generators,...

To help the cross-building environment, you can (and probably should, if things go wrong) annotate Build-Dependencies, so that the correct architecture is picked.

Declare "Indep" Build-Dependencies

Typically, cross-compilation is mostly interesting as a way to produce arch:any packages, as the host architecture can use arch:all packages produced on any (non-crossbuilding) build machine.

To allow the dependency resolver to not worry about any Build-Depends that it won't need anyhow, make sure to declare all build-dependencies that are only needed for building arch:all packages using the Build-Depends-Indep stanza.

Example:

   1 Build-Depends-Indep:
   2  doxygen,

Architecture qualifiers

What is required in package dependencies is for the depending source package to distinguish build-dependencies which are satisfiable by any architecture ('tools') from build-dependencies which can only be satisfied by packages of the same architecture ('libraries' generally). This is very similar to the Multi-Arch: field options 'foreign' (for tools) and 'same' (for libraries), respectively. However it's not exactly the same because the architecture relationship is defined by the depending package, not the depended-on package, because only the depending-on package knows what it needs the build-dependency for. This is recognised in the multiarch spec with the Multi-Arch: option 'allowed' and the Depends: package:any syntax.

Despite the relationship being 'from the wrong end', in practice it is almost always right to use the Multi-arch field to decide if the build or host version (or both) of a package should be installed. By marking the exceptions to this rule in a packages' build-dependencies we minimise the package metadata changes needed (most packages will need no changes to their Build-Depends for this reason).

Exceptions to the normal case are specified using the build-dependency qualifiers :native and :any.

Build Dependencies are resolved according to this table:

Build-Depends: foo

Build-Depends: foo:any

Build-Depends: foo:native

no Multi-Arch field

DEB_HOST_ARCH

disallowed

DEB_BUILD_ARCH

Multi-Arch: same

DEB_HOST_ARCH

disallowed

DEB_BUILD_ARCH

Multi-Arch: foreign

any, preferably DEB_BUILD_ARCH

disallowed

disallowed

Multi-Arch: allowed

DEB_HOST_ARCH

any, preferably DEB_BUILD_ARCH

DEB_BUILD_ARCH

See also Ubuntu/MultiarchCrossCompilers

Use ':any' qualifier for dependencies

For some Multi-Arch: allowed packages a given dependency must have the HOST architecture in one case, and the BUILD architecture in another case.

If you require your Multi-Arch: allowed Build-Dependency to be executable on the HOST machine, annotate this by adding the :any architecture qualifier:

   1 Depends:
   2  python3:any,

The :any qualifier is only valid for packages that are marked as Multi-Arch:allowed.

Use `dh-sequence-...` meta build-dependencies if possible

If you are using dh addons, it is often better to use a Build-Dependency on the relevant dh-sequence-... virtual package instead of invoking dh --with=... and manually adding Build-Dependencies.

Adding a dh-sequence-... B-D will:

The fun part is, that this also works when using Build-Depends-Indep. So the following will activate the sphinx sequence and install the relevant Build-Dependencies only when doing an arch:all build:

   1 Build-Depends-Indep:
   2  dh-sequence-sphinxdoc,

Hints for specific groups of packages

Perl extensions

Whenever building a Perl extension module (often called "xs" module), perl-xs-dev should be added to Build-Depends.

Python extensions

When building a Python extension, pythonSOMETHING-dev is often added to Build-Depends. For working with cross compilation, that should be replaced with libpythonSOMETHING-dev, pythonSOMETHING-dev:any.

Building

dh automatically takes care of most cross-building issues.

Setting the correct compiler

Simply hardcoding 'gcc' will not allow cross-building, and is arguably wrong for other reasons (like using llvm/clang instead).

The easiest way to re-initialize common tool variables inside debian/rules is relying on dpkg:

   1 include /usr/share/dpkg/buildtools.mk
   2 

Formerly, you could set CC manually:

CC=$(DEB_HOST_GNU_TYPE)-gcc

When dh deals with make build systems, using dh_auto_build will automatically pass cross tools as command variables.

When not using dh, things are a bit more complicated, as unfortunately make always sets $(CC) (to 'cc') so you cannot just test if it is already set, and set it if not.

This state can be detected with $(origin CC) see http://theory.uwinnipeg.ca/localfiles/infofiles/make/make_80.html.

In the end, this can be done this way for instance:

   1 DEB_HOST_GNU_TYPE   ?= $(shell dpkg-architecture -qDEB_HOST_GNU_TYPE)
   2 
   3 ifeq ($(origin CC),default)
   4 CC := $(DEB_HOST_GNU_TYPE)-gcc
   5 endif
   6 
   7 override_dh_auto_build:
   8     $(MAKE) CC=$(CC) build

Note: nowadays debhelper takes care of this, so these lines are usually not needed anymore.

This is a good way to set CC in a bash script: : "${CC:=gcc}"  which means if $CC is not already set, set it to 'gcc', otherwise use the existing value. The ':' is the 'null command' and stops the value being executed.

Splitting build sequence into `arch:any` and `arch:all`

As mentioned in #Build-Depends-Indep, it is often a good idea to separate the tasks of building arch:any packages (that need to be cross-compiled) and arch:all packages (where there is no need to do so).

Here's an example of a dh-based packaging build script, that uses sphinx only to generate an arch:all doc package:

   1 %-arch:
   2        dh $@ --buildsystem=cmake
   3 
   4 %-indep:
   5        dh $@ --with sphinxdoc --buildsystem=cmake
   6 
   7 clean:
   8        dh $@ --buildsystem=cmake
   9 
  10 execute_after_dh_auto_build-indep:
  11         dh_auto_build -- manual-html

See also #dh-sequence

Running pre-build tests/programs

pkg-config

Sometimes you need to run some programs directly within debian/rules (that is: 'outside' of dh), in which case you should make sure to pick the cross-building variants. You can get cross-building variants of common build tools by including the /usr/share/dpkg/buildtools.mk snippet.

To make everything more optional, the following example does a weak include and defines a fallback:

   1 -include /usr/share/dpkg/buildtools.mk
   2 PKG_CONFIG?=pkg-config
   3 
   4 export DEB_CFLAGS_MAINT_APPEND = $(shell $(PKG_CONFIG) --cflags foobar)

Autoconf configure tests

autoconf will not do tests that need a program to be run when in 'cross-compiling' mode. To get the results that would be missing we can use the config.cache mechanism to supply pre-seeded answers. These can be architecture-specific answers, or generic 'Debian' answers. In rare cases you may need to change them for your own use-case.

These autoconf cache files are managed by the dpkg-cross package, which contains a set of files in /etc/dpkg-cross/ to pre-seed the answers. cross-config.<arch> contains the architecture-specific stuff (anything depending on size of variables, lengths of pointers, or peculiarities of this arch's glibc implementation). cross-config.cache contains the architecture-independent stuff.

All you need to do in your package to enable this mechanism and use the default settings is to set the environment variable CONFIG_SITE=/etc/dpkg-cross/cross-config.$DEB_HOST_ARCH e.g. /etc/dpkg-cross/cross-config.armel.

If you need a special package-specific variable then you should set PACKAGE=<packagename> to match up with a stanza in one of the config files. This is to avoid clashes if two different packages need a variable set in a different way.

Running run time tests (test suite)

Classic cross-compiling assumes never running any host architecture binaries, and this remains a 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 'nocheck' by default, but it can be enabled when appropriate.

Running/skipping tests

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

set DEB_BUILD_OPTIONS=nocheck

However there are packages where the tests do make sense, and they can be done using qemu so the responsibility of packagers is just to honor this variable.

Where build profiles are enabled (DebianBootstrap) having a profile for 'check' is useful to annotate build-dependencies which are only needed for checks/tests.

Questions