How do I produce plain binary from object files?

Question

How should I produce raw binary file from two object (.o) files?

I want the plain binary format produced by nasm -f bin when compiling a .asm file, but for .o files.

By a plain binary, I mean a file which contains only the instructions, not some extra information, as many executable files contain a lot of extra helpful information.

See http://www.nasm.us/doc/nasmdoc7.html for information on that.

PS: I want to make a "plain binary" to start in QEMU.

All files are raw binary files. What do you actually want to do with the .o? — Tom Tanner, Aug 22 '14 at 08:05
What I want is that I have two object files one from gcc and one from nasm, I can link them to produce a executable but how to produce a raw binary ? — Pratik Singhal, Aug 22 '14 at 08:06
By raw binary I mean a plain binary without the details which are included in the executable files. — Pratik Singhal, Aug 22 '14 at 08:06
(I wonder who is upvoting this) "a plain binary without the details which are included in the executable files" still doesn't make sense. — wRAR, Aug 22 '14 at 08:10
@wRAR Check this :- http://stackoverflow.com/questions/1936844/what-is-the-difference-between-plain-binary-format-bin-and-windows-executable — Pratik Singhal, Aug 22 '14 at 08:12
@ps06756 and that question has answers that say mostly "there is no such thing". — wRAR, Aug 22 '14 at 08:13
I want the plain binary format produced by `nasm -f bin` when compiling a .asm file. — Pratik Singhal, Aug 22 '14 at 08:14
`I want the plain binary format produced by nasm -f bin when compiling a .asm file` Include that in your question — Don't You Worry Child, Aug 22 '14 at 08:43
And please explain why `nasm -f bin` is not [the correct procedure](http://www.nasm.us/doc/nasmdoc7.html). — Jongware, Aug 22 '14 at 08:49
@Jongware `nasm -f bin` produces plain binary from `.asm` source file, I want to produce the same thing from two `.o` object files. — Pratik Singhal, Aug 22 '14 at 09:55
It is a matter of invoking and scripting correctly the GNU `ld` binutils linker. BTW, plain binary executables cannot exist: you need at the very least a convention about the starting address and state (e.g. initial register contents). So please **edit your question** to explain why do you ask, and how would the produced "plain binary" be run (and be started) — Basile Starynkevitch, Aug 22 '14 at 10:28
I guess he wants a machine readable code, to run bare metal (without operating system) — Vitor, Aug 22 '14 at 13:25

Christoph · Accepted Answer · 2014-08-23T09:02:47.343

This brings back memories. I'm sure there is a better way to do this with linker scripts, but this is how I did it when I was young and stupid:

# compile some files
gcc -c -nostdlib -nostartfiles -nodefaultlibs -fno-builtin kernel.c -o kernel.o
gcc -c -nostdlib -nostartfiles -nodefaultlibs -fno-builtin io.c -o io.o

# link files and place code at known address so we can jump there from asm
ld -Ttext 0x100000 kernel.o io.o -o kernel.out

# get a flat binary
objcopy -S -O binary kernel.out kernel.bin

The file kernel.c started with

__asm__("call _kmain");
__asm__("ret");

void kmain(void) { ... }

The fun part is writing the loader in assembler.

score 1 · Answer 2 · edited May 23 '17 at 11:45

ld --oformat binary is a more direct option:

ld --oformat binary -o main.img -Ttext 0x7C00 main.o

The downside of this method is that I don't think it is possible to reuse the symbols to debug, as we'd want something like:

qemu-system-i386 -hda main.img -S -s &
gdb main.elf -ex 'target remote localhost:1234'

So in that case you should stick to objcopy. See also: https://stackoverflow.com/a/32960272/895245

Also make sure that you use your own clean linker script: https://stackoverflow.com/a/32594933/895245

Repository with working examples for some common cases:

Similar question: How to generate plain binaries like nasm -f bin with the GNU GAS assembler?

How do I produce plain binary from object files?

2 Answers2