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Generate raw binary from C code in Linux

I have been implementing just for fun a simple operating system for x86 architecture from scratch. I implemented the assembly code for the bootloader that loads the kernel from disk and enters in 32-bit mode. The kernel code that is loaded is written in C, so in order to be executed the idea is to generate the raw binary from the C code.

Firstly, I used these commands:

$gcc -ffreestanding -c kernel.c -o kernel.o -m32

$ld -o kernel.bin -Ttext 0x1000 kernel.o --oformat binary -m elf_i386

However, it didn’t generate any binary giving back these errors:

kernel.o: In function 'main':
kernel.c:(.text+0xc): undefined reference to '_GLOBAL_OFFSET_TABLE_'

Just for clarity sake, the kernel.c code is:

/* kernel.c */

void main ()
{
   char *video_memory = (char *) 0xb8000 ;
   *video_memory = 'X';
}

Then I followed this tutorial: http://wiki.osdev.org/GCC_Cross-Compiler to implement my own cross-compiler for my own target. It worked for my purpose, however disassembling with the command ndisasm I obtained this code: 

00000000  55                push ebp
00000001  89E5              mov ebp,esp
00000003  83EC10            sub esp,byte +0x10
00000006  C745FC00800B00    mov dword [ebp-0x4],0xb8000
0000000D  8B45FC            mov eax,[ebp-0x4]
00000010  C60058            mov byte [eax],0x58
00000013  90                nop
00000014  C9                leave
00000015  C3                ret
00000016  0000              add [eax],al
00000018  1400              adc al,0x0
0000001A  0000              add [eax],al
0000001C  0000              add [eax],al
0000001E  0000              add [eax],al
00000020  017A52            add [edx+0x52],edi
00000023  0001              add [ecx],al
00000025  7C08              jl 0x2f
00000027  011B              add [ebx],ebx
00000029  0C04              or al,0x4
0000002B  0488              add al,0x88
0000002D  0100              add [eax],eax
0000002F  001C00            add [eax+eax],bl
00000032  0000              add [eax],al
00000034  1C00              sbb al,0x0
00000036  0000              add [eax],al
00000038  C8FFFFFF          enter 0xffff,0xff
0000003C  16                push ss
0000003D  0000              add [eax],al
0000003F  0000              add [eax],al
00000041  41                inc ecx
00000042  0E                push cs
00000043  088502420D05      or [ebp+0x50d4202],al
00000049  52                push edx
0000004A  C50C04            lds ecx,[esp+eax]
0000004D  0400              add al,0x0
0000004F  00                db 0x00

As you can see, the first 9 rows (except for the NOP that I don’t know why it is inserted) are the assembly translation of my main function. From 10 row to the end, there’s a lot code that I don’t know why it is here.

In the end, I have two questions:

1) Why is it produced that code?

2) Is there a way to produce the raw machine code from C without that useless stuff?

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Answer

A few hints first:

  • avoid naming your starting routine main. It is confusing (both for the reader and perhaps for the compiler; when you don’t pass -ffreestanding to gcc it is handling main very specifically). Use something else like start or begin_of_my_kernel …

  • compile with gcc -v to understand what your particular compiler is doing.

  • you probably should ask your compiler for some optimizations and all warnings, so pass -O -Wall at least to gcc

  • you may want to look into the produced assembler code, so use gcc -S -O -Wall -fverbose-asm kernel.c to get the kernel.s assembler file and glance into it

  • as commented by Michael Petch you might want to pass -fno-exceptions

  • your probably need some linker script and/or some hand-written assembler for crt0

  • you should read something about linkers & loaders

 kernel.c:(.text+0xc): undefined reference to '_GLOBAL_OFFSET_TABLE_'

This smells like something related to position-independent-code. My guess: try compiling with an explicit -fno-pic or -fno-pie

(on some Linux distributions, their gcc might be configured with some -fpic enabled by default)

PS. Don’t forget to add -m32 to gcc if you want x86 32 bits binaries.

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