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I have a 64-bit (amd64 a.k.a. x86_64) Debian or Ubuntu installation. I need to run 32-bit (i386/i686) programs occasionally, or to compile programs for a 32-bit system. How can I do this with a minimum of fuss?

Bonus: what if I want to run or test with an older or newer release of the distribution?

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2 Answers 2

up vote 21 down vote accepted

Native support

Since Ubuntu 11.04 and Debian wheezy (7.0), Debian and Ubuntu have multiarch support: you can mix x86_32 (i386) and x86_64 (amd64) packages on the same system in a straightforward way. This is known as multiarch support See warl0ck's answer for more details.

In older releases, Debian and Ubuntu ship with a number of 32-bit libraries on amd64. Install the ia32-libs Install ia32-libs package to have a basic set of 32-bit libraries, and possibly other packages that depend on this one. Your 32-bit executables should simply run if you have all the required libraries. For development, install gcc-multilib Install gcc-multilib, and again possibly other packages that depend on it such as g++-multilib. You may find binutils-multiarch Install binutils-multiarch useful as well, and ia32-libs-dev on Debian. Pass the -m32 option to gcc to compile for ix86.

Note that uname -m will still show x64_64 if you're running a 64-bit kernel, regardless of what 32-bit user mode components you have installed. Schroot described below takes care of this.

Schroot

This section is a guide to installing a Debian-like distribution “inside” another Linux distribution. It is worded in terms of installing a 32-bit Ubuntu inside a 64-bit Ubuntu, but should apply with minor modifications to other situations, such as installing Debian unstable inside Debian stable or vice versa.

Introduction

The idea is to install an alternate distribution in a subtree and run from that. You can install a 32-bit system on a 64-bit system that way, or a different release of your distribution, or a testing environment with different sets of packages installed.

The chroot command and system call starts a process with a view of the filesystem that's restricted to a subtree of the directory tree. Debian and Ubuntu ship schroot, a utility that wraps around this feature to create a more usable sub-environment.

Install the schroot package Install schroot (Debian) and the debootstrap package Install debootstrap (Debian). Debootstrap is only needed for the installation of the alternate distribution and can be removed afterwards.

Set up schroot

This example describes how to set up a 32-bit Ubuntu 10.04LTS (lucid lynx) alternate environment. A similar setup should work with other releases of Debian and Ubuntu. Create a file /etc/schroot/chroot.d/lucid32 with the following contents:

[lucid32]
description=Ubuntu 10.04LTS 32-bit
directory=/32
type=directory
personality=linux32
users=yourusername
groups=users,admin

The line directory=/32 tells schroot where we'll put the files of the 32-bit installation. The line username=yourusername says the user yourusername will be allowed to use the schroot. The line groups=users,admin says that users in either group will be allowed to use the schroot; you can also put a users=… directive.

Install the new distribution

Create the directory and start populating it with debootstrap. Debootstrap downloads and installs a core set of packages for the specified distribution and architecture.

mkdir /32
debootstrap --arch i386 lucid /32 http://archive.ubuntu.com/ubuntu

You almost have a working system already; what follows is minor enhancements. Schroot automatically overwrites several files in /32/etc when you run it, in particular the DNS configuration in /etc/resolv.conf and the user database in /etc/passwd and other files (this can be overridden, see the documentation). There are a few more files you may want to copy manually once and for all:

cp -p /etc/apt/apt.conf /32/etc/apt/      # for proxy settings
cp -p /etc/apt/sources.list /32/etc/apt/  # for universe, security, etc
cp -p /etc/environment /32/etc/           # for proxy and locale settings
cp -p /etc/sudoers /32/etc/               # for custom sudo settings

There won't be a file /etc/mtab or /etc/fstab in the chroot. I don't recommend using the mount command manually in the chroot, do it from outside. But do create a good-enough /etc/mtab to make commands such as df work reasonably.

ln -s /proc/mounts /32/etc/mtab

With the directory type, schroot will perform bind mounts of a number of directories, i.e. those directories will be shared with the parent installation: /proc, /dev, /home, /tmp.

Services in the chroot

As described here, a schroot is not suitable for running daemons. Programs in the schroot will be killed when you exit the schroot. Use a “plain” schroot instead of a “directory” schroot if you want it to be more permanent, and set up permanent bind mounts in /etc/fstab on the parent installation.

On Debian and Ubuntu, services start automatically on installation. To avoid this (which could disrupt the services running outside the chroot, in particular because network ports are shared), establish a policy of not running services in the chroot. Put the following script as /32/usr/sbin/policy-rc.d and make it executable (chmod a+rx /32/usr/sbin/policy-rc.d).

#!/bin/sh
## Don't start any service if running in a chroot.
## See /usr/share/doc/sysv-rc/README.policy-rc.d.gz
if [ "$(stat -c %d:%i /)" != "$(stat -c %d:%i /proc/1/root/.)" ]; then
  exit 101
fi

Populate the new system

Now we can start using the chroot. You'll want to install a few more packages at this point.

schroot -c lucid32
sudo apt-get update
apt-get install lsb-core nano
...

You may need to generate a few locales, e.g.

locale-gen en_US en_US.utf8

If the schroot is for an older release of Ubuntu such as 8.04 (hardy), note that the package ubuntu-standard pulls in an MTA. Select nullmailer instead of the default postfix (you may want your chroot to send mail but you definitely don't want it to receive any).

Going further

For more information, see the schroot manual, the schroot FAQ and the schroot.conf manual. Schroot is part of the Debian autobuilder (buildd) project. There may be additional useful tips on the Ubuntu community page about debootstrap.

Virtual machine

If you need complete isolation of the alternate environment, use a virtual machine such as KVM (qemu-kvm Install qemu-kvm) or VirtualBox.

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You might want to edit this to include multiarch info. –  terdon Feb 25 at 12:20
    
@terdon Done. I'm keeping this answer up because the schroot method is useful in many other cases, e.g. LTS + point release, stable + testing, … –  Gilles Feb 25 at 12:47
    
Absolutely, we've just been discussing this in chat and wanted to get an up to date version as well, do not remove the schroot info! That's distro agnistic and very useful. –  terdon Feb 25 at 12:48
    
@Gilles Can you add an explanation of what the [ "$(stat -c %d:%i /)" != "$(stat -c %d:%i /proc/1/root/.)" ] condition does? –  Faheem Mitha Jun 20 at 12:37
    
@FaheemMitha It tests whether the script is running in a chroot. It's there to support booting the chroot installation as a standalone system. –  Gilles Jun 20 at 13:01

Since Ubuntu 11.04 (natty) and Debian 7.0 (wheezy) introduced multiarch support, 32-bit and 64-bit libraries can coexist on one system. To install a 32-bit library libXX, first add the necessary 32bit architecture to your system:

For Ubuntu:

sudo dpkg --foreign-architecture i386

And for Debian:

sudo dpkg --add-architecture i386

Then install the 32bit library:

sudo apt-get install libXX:i386

The ia32-libs package. As of Ubuntu 12.04 precise, it no longer contains any libraries, it only pulls in libXX:i386 packages as dependencies.

The compile part is rather easy for C and C++ programs, add -m32 to CFLAG or CXXFLAG, which will make generated program 32bit, e.g

export CFLAGS="-m32"
gcc main.c -o main

This works for makefile-based projects as well.

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2  
apt-get install ia32-libs worked for me (13.04)! –  Étienne May 12 '13 at 19:43

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