1.8 KiB
layout: post title: "C headers in Asm" subtitle: "Cleaning up the build chain" tags: [osdev]
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
EXACT_PI equ 3
{: .prettyprint .lang-nasm}
and in c
#define EXACT_PI 3
{: .prettyprint .lang-c} 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
#include <header.h>
mov eax, EXACT_PI
{: .prettyprint .lang-nasm}
include/header.h
#pragma once
#define EXACT_PI 3
#ifndef __ASSEMBLER__
// This is not evaluated if header.h is included from an assembly file.
#endif
{: .prettyprint .lang-c}
This is compiled through:
cpp -I include -x assembler-with-cpp myAsmFile.asm -o myAsmFile.s
nasm myAsmFile.s
{: .prettyprint}
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.