What is this Multiarch?
Multiarch lets you install library packages from multiple architectures on the same machine. This is useful in various ways, but the most common is installing both 64 and 32-bit software on the same machine and having dependencies correctly resolved automatically. In general you can have libraries of more than one architecture installed together and applications from one architecture or another installed as alternatives. Note that it does not enable multiple architecture versions of applications to be installed simultaneously.
There is a current machine architecture, as printed by dpkg --print-architecture. It is built-in to the currently installed dpkg package.
( Note that architecture here actually refers to an 'ABI' (Application Binary Interface), not an instruction set (ISA). So for example, armel and armhf are different architectures, even though they use (near enough) the same instruction set, because they have different library-calling ABIs. )
Packages can now be specified as 'package:architecture' pretty-much anywhere that was previously just 'package', so we have libc:i386 and libc:amd64, unfortunately the semantics in dpkg and apt are slightly different so you might get different results, but it should always be safe and unambiguous to arch-qualify packages. The bare name 'package' refers to the current machine architecture in apt.
Other available architectures are shown by dpkg --print-foreign-architectures. dpkg will manage packages for these architectures as well as the machine architecture.
There is a 'Multi-Arch' header in the package metadata of any multiarch-aware package.
Existing packages work fine in a multiarch environment, just as before, but to gain the benefits of co-installation or cross-architecture dependencies, many packages need to be made 'multiarch-aware'.
- For an unchanged package you can choose which arch version of a package to install (e.g. 'amd64' or 'i386').
- If a package is marked 'Multi-Arch: foreign', then it can satisfy dependencies of a package of a different architecture (e.g 'make:amd64' will satisfy a dependency on make for any-architecture package).
- To enable more than one architecture version of a package to be installed at the same time (generally libraries and dev- packages) files need to be moved so they don't clash. These packages are marked 'Multi-Arch: same'.
Packages marked 'Multi-Arch : allowed also exist which can be treated as either :same or :foreign depending on how they are depended-on.
Packagers are currently working through the distro, starting with the most useful packages for making multi-arch aware. See the multiarch spec and implementation howto for details of how it all actually works, and how to update packages to take advantage of the functionality.
You need a multiarch-aware dpkg and apt.
In Debian dpkg this is present since 1.16.2. In Ubuntu this is present since natty (v220.127.116.11ubuntu1). Check by seeing if dpkg --print-foreign-architectures is understood.
Apt is multiarch-aware if it supports -o APT::Architectures. This is available from version 0.8.13 onwards. However there are many multiarch-related improvements and bug-fixes in later apt versions (some required by Debian dpkg 1.16.2 to properly enable multiarch), such as apt-get build-dep -a cross-dependency support, so the later the better in general up to at least 0.9.4.
Prior to apt 0.9 in Debian, dpkg can get stuck (but only if multiarch is enabled) during upgrades when it is not told which arch package it should be configuring by apt. (dpkg: error: --configure needs a valid package name but 'gcc-4.7-base' is not: ambiguous package name 'gcc-4.7-base' with more than one installed instance) dpkg --configure -a will unbung this.
To add an extra architecture (in Debian from dpkg 1.16.2 onwards):
dpkg --add-architecture <arch>
dpkg --add-architecture armhf
To remove an architecture
dpkg --remove-architecture <arch>
dpkg architectures are stored in /var/lib/dpkg/arch.
Note that the Ubuntu dpkg in natty (1.16.0~ubuntu7 (reports 18.104.22.168)), oneiric and precise (22.214.171.124ubuntu7) uses a different syntax:
echo "foreign-architecture armhf" > /etc/dpkg/dpkg.cfg.d/architectures
Setting up apt sources
Apt defaults to using the set of architectures reported by dpkg, and any unqualified architecture deb lines in /etc/apt/sources.list, which is usually what you wanted. This can be overridden using APT::Architecture=<arch> to set the default architecture or APT::Architectures="<arch> <arch>".
apt-sources can be architecture qualified with this syntax. This is very useful on Ubuntu's split archive. It is not normally necessary on Debian unless your normal archive does not mirror the extra architectures you are interested in.
deb [arch=amd64,i386] http://uk.archive.ubuntu.com/ubuntu/ quantal main universe deb [arch=armel,armhf] http://ports.ubuntu.com/ubuntu-ports quantal main universe
Arch-qualifying deb-src lines doesn't make any sense.
Note: There is a bug in apt versions >=0.9.7 and <0.9.7.2 which means that putting 'arch=armel,armhf' on one line didn't work - you needed two separate entries.
Don't forget to apt-get update after adding new architectures.
To install a package of the non-default architecture just specify that architecture on the command line:
apt-get install package:architecture
That package's dependencies will be installed automatically for the correct architectures (same-arch library deps, machine-arch for other deps) e.g
apt-get install links:i386
dpkg -i package_version_architecture.deb dpkg -r package:architecture
To install build-dependencies of a package before cross-building:
apt-get build-dep -a <arch> <package>
This only works when all the 'tools' packages depended-on are marked Multi-Arch: foreign, any depended-on libraries which are also needed on the BUILD machine, and -dev packages which are needed for both HOST and BUILD architectures are made co-installable ('Multi-Arch: same' with arch-qualified paths), and any exceptions to the default rules are marked package:any or package:native in the package source. This process is ongoing.
When it doesn't work you can often get the dependencies installed with a manual apt-get line: e.g instead of
apt-get build-dep -a armhf acl
apt-get install autoconf automake debhelper gettext libtool libattr1-dev:armhf
Details of how this resolves are on MultiarchCross.