Initial commit - Begun rewrite of website generator

pages/2012-06-18-C-Headers-In-Asm.md

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+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](https://github.com/thomasloven/os5/tree/742f2348ecc58eaa8239b06c666bd8c3c539c019).