layout: post
title: "C headers in Asm"
subtitle: "Cleaning up the build chain"
tags: [osdev]

> **NOTE (2016-11-01)**
>
> Since people are apparently still finding this page four years later (yay! Cool URLs don't change!): I've since found a better and more correct way of doing this.
>
> If you're compiling using `gcc`, you can just name your assembly files (with includes and macros and stuff) `whatever.S` (capital S), and compile them right down to `whatever.o` as you normally would (using `gcc`, not `as`).
>
> GNU make also has a builtin rule that does this automatically... yeah...
>
> From this I've learned the following
>
> - Trust make. It's terribly powerful if you trust it to be.
> - GCC is not the GNU C Compiler. It's the GNU Compiler Collection.

Something that always annoyed me is how hard it is to synchronize constants
between assembly and c code.  In assembler, you define a constant value as

    :::nasm
    EXACT_PI equ 3

and in c

    :::c
    #define EXACT_PI 3

As is usually the case with things that annoy me, there is of course a solution
to this, as I found out today.  The solution is the c preprocessor.

Normally, when you run a c compiler, it makes multiple passes over your source
file. The first one or two times, it runs a pre-processor. The preprocessor
checks for things like _#include_ and _#define_ and replaces macros. The next
pass actually compiles the code. Then the compiler invokes a linker and so on.

What I found out today is that you can run only the preprocessor and it will
replace all the preprocessor code and ignore the rest.  In other words, you can
use c preprocessor macros in assembler. Awesome!

So, how is this done?
Well, here's a minimal (non-working) example:

_myAsmFile.asm_

    :::nasm
    #include <header.h>

    mov eax, EXACT_PI

_include/header.h_

    :::c
    #pragma once

    #define EXACT_PI 3

    #ifndef __ASSEMBLER__
    // This is not evaluated if header.h is included from an assembly file.
    #endif

This is compiled through:

    :::bash
    $ cpp -I include -x assembler-with-cpp myAsmFile.asm -o myAsmFile.s
    $ nasm myAsmFile.s

The _-x_-flag tells the preprocessor what type of file the following input
files are. _assembler-with-cpp_ means _cpp_ will ignore everything but the
preprocessor commands.

An alternative to _cpp_ is _gcc -E_. Actually, this is often exactly the same
thing...


This is implemented in git commit [742f2348ec](https://github.com/thomasloven/os5/tree/742f2348ecc58eaa8239b06c666bd8c3c539c019).