How to Compile 32-bit Apps on 64-bit Ubuntu?

This is known to work on Ubuntu 16.04 through 22.04:

sudo apt install gcc-multilib g++-multilib

Then a minimal hello world:

main.c

#include <stdio.h>

int main(void) {
    puts("hello world");
    return 0;
}

compiles without warning with:

gcc -m32 -ggdb3 -O0 -pedantic-errors -std=c89 \
  -Wall -Wextra -pedantic -o main.out main.c

And

./main.out

outputs:

hello world

And:

file main.out

says:

main.out: ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=87c87a83878ce7e7d23b6236e4286bf1daf59033, not stripped

and:

qemu-i386 main.out

also gives:

hello world

but fails on an x86_64 executable with:

./main.out: Invalid ELF image for this architecture

Furthermore, I have:

• run the compiled file in a 32 bit VM
• compiled and run an IA-32 C driver + complex IA-32 code

So I think it works :-)

See also: Cannot find crtn.o, linking 32 bit code on 64 bit system

It is a shame that this package conflicts with the cross compilers like gcc-arm-linux-gnueabihf https://bugs.launchpad.net/ubuntu/+source/gcc-defaults/+bug/1300211

Running versions of the question:

• https://unix.stackexchange.com/questions/12956/how-do-i-run-32-bit-programs-on-a-64-bit-debian-ubuntu
• https://askubuntu.com/questions/454253/how-to-run-32-bit-app-in-ubuntu-64-bit

We are able to run 32-bit programs directly on 64-bit Ubuntu because the Ubuntu kernel is configured with:

CONFIG_IA32_EMULATION=y

according to:

grep CONFIG_IA32_EMULATION "/boot/config-$(uname -r)"

whose help on the kernel source tree reads:

Include code to run legacy 32-bit programs under a
64-bit kernel. You should likely turn this on, unless you're
100% sure that you don't have any 32-bit programs left.

This is in turn possible because x86 64 bit CPUs have a mode to run 32-bit programs that the Linux kernel uses.

TODO: what options does gcc-multilib get compiled differently than gcc?

To get Ubuntu Server 12.04 LTS 64-bit to compile gcc 4.8 32-bit programs, you'll need to do two things.

1. Make sure all the 32-bit gcc 4.8 development tools are completely installed:

   sudo apt-get install lib32gcc-4.8-dev

2. Compile programs using the -m32 flag

   gcc pgm.c -m32 -o pgm

Multiarch installation is supported by adding the architecture information to the package names you want to install (instead of installing these packages using alternative names, which might or might not be available).

See this answer for more information on (modern) multiarch installations.

In your case you'd be better off installing the 32bit gcc and libc:

sudo apt-get install libc6-dev:i386 gcc:i386

It will install the 32-bit libc development and gcc packages, and all depending packages (all 32bit versions), next to your 64-bit installation without breaking it.