Linkage of standard libraries in C++ code called from ASM

Question

as I am developing my "OsDev" project, where I am learning a new stuff (for somebody who did not code in C/C++ for a long time due to web development it is kinda "new"). I figured out in the other thread, that calling a C++ function from ASM needs to have a extern "C" prefix but now I have problem with the lining of standard libraries as a for example cstdio etc. I stuck with this message.

kc.o: In function `kmain':
kernel.cpp:(.text+0x3e4): undefined reference to `strlen`

C++

#include <string.h>
#include <cstdio>

#include "inc/screen.h"

extern "C" void kmain()
{
    clearScreen();
    kernel_print((char*)"Hello Github! :-)", 0x04);
}

and if I try to use strlen() it won't link. (BTW. including screen.h is working for some reason). Compiling script

nasm -f elf32 kernel.asm -o kasm.o
g++ -c kernel.cpp -o kc.o -lgcc -m32  -Wall -Wextra -O2
ld -m elf_i386 -T link.ld -o kernel.bin kasm.o kc.o

link.ld

OUTPUT_FORMAT(elf32-i386)
ENTRY(start)
SECTIONS
 {
   . = 0x100000;
   .text : { *(.text) }
   .data : { *(.data) }
   .bss  : { *(.bss)  }
 }

Thanks for any suggestions. :)

Answer 1

Your code cannot work as kernel's can't directly use shared libraries.

Why can't I use shared libraries directly in my kernel?

When an application is loaded by the operating system, all the required files are brought into its address space. This includes the executable file and any dynamic libraries (all ABI-conforming ELF applications will always link with a system library - the C Standard Library or just libc ).

But while loading the kernel, only the original executable is loaded. Multiboot 2 (with GRUB bootloader) will allow you to load kernel-modules which can be dynamic libraries. But still, your kernel must know how to link itself and the kernel-modules in physical memory. To do so, you must implement a ELF parser and dynamic linker in your kernel.

Before implementing one, make sure your kernel is mature enough to systematically handle dynamic memory allocation, pagination, and other basic features.

How can I use the sweet features of libc?

Usually, you won't use all of the userspace functionality of libc . But things like memcpy , strlen , strcpyn and so on are absolutely necessary. You will have to implement these functions on your own, but the better part here is that, you can change the names of these functions. For example, if you prefere camelCase for function names, then you can also use function names like copyMemory , lengthOfString , etc.

https://github.com/SukantPal/Silcos-Kernel

I have built my own kernel, which has a few implementations of the required functions in KernelHost/Source/Util/CircuitPrimitive.cpp . You can look into that. Also, it has a full-fledged module linker. (KernelHost, ModuleFramework, etc. those parent folders contain separate kernel-module source code).

Make sure not to use the standard C headers in your kernel, as for now. Implement all required functions on your own, including printf