Compiling C Programs with GCC

On 64-bit Linux, GCC compiles a C program by default to a 64-bit ELF executable. To compile as 32-bit, you first need the GCC multilib support package. For example:

sudo apt install gcc-multilib.

Then you need to use -m32 as an argument to GCC. So, if a 64-bit executable is created with:

gcc -o program-64 program.c

you can compile it 32-bit using:

gcc -m32 -o program-32 program.c

Compiling Assembly Programs with NASM

Unlike C, assembly is not portable between 32-bit and 64-bit systems, so the source files to NASM will be different. NASM also must be told explicitly which object format to write. So for 32-bit output, use:

nasm -f elf32 program-32.asm

and for 64-bit output, use:

nasm -f elf64 program-64.asm

The resultant object files need to be linked into an appropriate executable format. To create a 32-bit ELF, use:

ld -m elf_i386 -s -o program-32 program-32.o

and a 64-bit ELF can be created using the following:

ld -m elf_x86_64 -s -o program-64 program-64.o