Generating a.out file format with GCC

Question

How do I generate the a.out file format with GCC on x86 architectures?

With NASM I can do this easily with the -f flag, for example:

nasm -f aout start.asm
objdump -a start.o

start.o:     file format a.out-i386-linux
start.o

On Linux, compiling .c files produces an ELF object file. How can I produce a.out files with GCC?

Solution 1

To generate the a.out format with gcc, your linker needs to be told to do so. You can do it by passing it flags from gcc thanks to the -Wl flag.

Here is what you would do for the a.out format:

gcc -Wl,--oformat=a.out-i386-linux file.c -o file.out

You can also display all formats supported by typing:

objdump -i

Solution 2

According to the post Re: How can I control the gcc's output format?, you need to build gcc for a different target (i386-aout).

It sounds plausible as a.out has been deprecated for years (10+).

Comments

[Nulik]

I am developing an operating system and I need kernel and applications to be packed together in one image file so it is loaded quickly with one read and one write (the whole image) , a.out looks to be the ideal format for this, being simple and supported by a lot of bootloaders.

[Nulik]

the problem is, I have many object files (.o) which are in elf format, so when I link them with OUTPUT_FORMAT(a.out-i386-linux) parameter in the linker script i get this error: "can not represent section `.comment' in a.out object file format". This is why I need the "gcc -c file.c" being compiled from the beginning into a.out format, and not ELF.

[Nulik]

I found the solution to the above, with -fno-ident option .comment section wasn't generated, i booted my a.out kernel fine with grub. thanks

[Quentin Casasnovas]

Sorry I missed your first comment, and glad that you found the fix yourself :)