Simple kernel compilation problems

Question

I'm having problems with linking two .o files with gcc compiler. I have my bootloader which is written in assembly and c++ object which it's intended to load. When linking these two files the compiler complains about undefined reference to kernel_main, and I can't figure out why...

    //kernel.cpp
    #include <stddef.h> //we can use it: it doesnt use any platform-related api             functions
    #include <stdint.h> //include it to get int16_t and some integer types

   /* Hardware text mode color constants. */
   enum vga_color{

   COLOR_BLACK = 0,
   COLOR_BLUE = 1,
   COLOR_GREEN = 2,
   COLOR_CYAN = 3,
   COLOR_RED = 4,
   COLOR_MAGENTA = 5,
   COLOR_BROWN = 6,
   COLOR_LIGHT_GREY = 7,
   COLOR_DARK_GREY = 8,
   COLOR_LIGHT_BLUE = 9,
   COLOR_LIGHT_GREEN = 10,
   COLOR_LIGHT_CYAN = 11,
   COLOR_LIGHT_RED = 12,
   COLOR_LIGHT_MAGENTA = 13,
   COLOR_LIGHT_BROWN = 14,
   COLOR_WHITE = 15,
 };

uint8_t make_color(enum vga_color fg, enum vga_color bg)
{
     return fg | bg << 4;
}

uint16_t make_vgaentry(char c, uint8_t color)
{
    uint16_t c16 = c;
    uint16_t color16 = color;
    return c16 | color16 << 8;
}

size_t strlen(const char* str)
{
   size_t ret = 0;
   while ( str[ret] != 0 )
   ret++;
   return ret;
}

 static const size_t VGA_WIDTH = 80;
 static const size_t VGA_HEIGHT = 24;

 size_t terminal_row;
 size_t terminal_column;
 uint8_t terminal_color;
 uint16_t* terminal_buffer;

 void terminal_initialize()
 {
    terminal_row = 0;
    terminal_column = 0;
    terminal_color = make_color(COLOR_LIGHT_GREY, COLOR_BLACK);
    terminal_buffer = (uint16_t*) 0xB8000;
    for ( size_t y = 0; y < VGA_HEIGHT; y++ )
    {
        for ( size_t x = 0; x < VGA_WIDTH; x++ )
        {
            const size_t index = y * VGA_WIDTH + x;
            terminal_buffer[index] = make_vgaentry(' ', terminal_color);
        }
    }
}

void terminal_setcolor(uint8_t color)
{
    terminal_color = color;
}

void terminal_putentryat(char c, uint8_t color, size_t x, size_t y)
{
    const size_t index = y * VGA_WIDTH + x;
    terminal_buffer[index] = make_vgaentry(c, color);
}

void terminal_putchar(char c)
{
    terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
    if ( ++terminal_column == VGA_WIDTH )
    {
        terminal_column = 0;
        if ( ++terminal_row == VGA_HEIGHT )
        {
            terminal_row = 0;
        }
    }
}

void terminal_writestring(const char* data)
{
    size_t datalen = strlen(data);
    for ( size_t i = 0; i < datalen; i++ )
        terminal_putchar(data[i]);
}

extern "C" void kernel_main()
{
terminal_initialize();
terminal_writestring("wellcome to my first operating system!");
for(;;);
}

Heres my assembly:

; Declare constants used for creating a multiboot header.
 MBALIGN     equ  1<<0                   ; align loaded modules on page boundaries
 MEMINFO     equ  1<<1                   ; provide memory map
 FLAGS       equ  MBALIGN | MEMINFO      ; this is the Multiboot 'flag' field
 MAGIC       equ  0x1BADB002             ; 'magic number' lets bootloader find the    header
 CHECKSUM    equ -(MAGIC + FLAGS)        ; checksum of above, to prove we are multiboot

     ; Declare a header as in the Multiboot Standard. We put this into a special
     ; section so we can force the header to be in the start of the final program.
     ; You don't need to understand all these details as it is just magic values that
     ; is documented in the multiboot standard. The bootloader will search for this
     ; magic sequence and recognize us as a multiboot kernel.
     section .multiboot
     align 4
        dd MAGIC
        dd FLAGS
        dd CHECKSUM

        ; Currently the stack pointer register (esp) points at anything and using it may
        ; cause massive harm. Instead, we'll provide our own stack. We will allocate
        ; room for a small temporary stack by creating a symbol at the bottom of it,
        ; then allocating 16384 bytes for it, and finally creating a symbol at the top.
        section .bootstrap_stack
        align 4
        stack_bottom:
        times 16384 db 0
        stack_top:

        ; The linker script specifies _start as the entry point to the kernel and the
        ; bootloader will jump to this position once the kernel has been loaded. It
        ; doesn't make sense to return from this function as the bootloader is gone.
        section .text
        global _start
        _start:
    ; Welcome to kernel mode! We now have sufficient code for the bootloader to
    ; load and run our operating system. It doesn't do anything interesting yet.
    ; Perhaps we would like to call printf("Hello, World\n"). You should now
    ; realize one of the profound truths about kernel mode: There is nothing
    ; there unless you provide it yourself. There is no printf function. There
    ; is no <stdio.h> header. If you want a function, you will have to code it
    ; yourself. And that is one of the best things about kernel development:
    ; you get to make the entire system yourself. You have absolute and complete
    ; power over the machine, there are no security restrictions, no safe
    ; guards, no debugging mechanisms, there is nothing but what you build.

    ; By now, you are perhaps tired of assembly language. You realize some
    ; things simply cannot be done in C, such as making the multiboot header in
    ; the right section and setting up the stack. However, you would like to
    ; write the operating system in a higher level language, such as C or C++.
    ; To that end, the next task is preparing the processor for execution of
    ; such code. C doesn't expect much at this point and we only need to set up
    ; a stack. Note that the processor is not fully initialized yet and stuff
    ; such as floating point instructions are not available yet.

    ; To set up a stack, we simply set the esp register to point to the top of
    ; our stack (as it grows downwards).
    mov esp, stack_top

    ; We are now ready to actually execute C code. We cannot embed that in an
    ; assembly file, so we'll create a kernel.c file in a moment. In that file,
    ; we'll create a C entry point called kernel_main and call it here.
    extern kernel_main
    call kernel_main

    ; In case the function returns, we'll want to put the computer into an
    ; infinite loop. To do that, we use the clear interrupt ('cli') instruction
    ; to disable interrupts, the halt instruction ('hlt') to stop the CPU until
    ; the next interrupt arrives, and jumping to the halt instruction if it ever
    ; continues execution, just to be safe.
    cli
        .hang:
    hlt
    jmp .hang

My linker script linker.ld

/* The bootloader will look at this image and start execution at the symbol

designated as the entry point. */ ENTRY(_start)

/* Tell where the various sections of the object files will be put in the final
   kernel image. */
SECTIONS
{
    /* Begin putting sections at 1 MiB, a conventional place for kernels to be
       loaded at by the bootloader. */
    . = 1M;

    /* First put the multiboot header, as it is required to be put very early
       early in the image or the bootloader won't recognize the file format.
       Next we'll put the .text section. */
    .text BLOCK(4K) : ALIGN(4K)
    {
        *(.multiboot)
        *(.text)
    }

    /* Read-only data. */
.rodata BLOCK(4K) : ALIGN(4K)
{
    *(.rodata)
}

/* Read-write data (initialized) */
.data BLOCK(4K) : ALIGN(4K)
{
    *(.data)
}

/* Read-write data (uninitialized) and stack */
.bss BLOCK(4K) : ALIGN(4K)
{
    *(COMMON)
    *(.bss)
    *(.bootstrap_stack)
}

/* The compiler may produce other sections, by default it will put them in
   a segment with the same name. Simply add stuff here as needed. */

} Both of these codes compile and I'm not getting any errors until I have to link them together. Any help would be appreciated :). EDIT: My gcc is running on 64bit ubuntu in virtual box. I compile the assembly file with

nasm -felf boot.asm -o boot.o

And for the kernel.cpp I use

g++ -m32 -c kernel.cpp -o kernel.o -ffreestanding -O2 -Wall -Wextra -fno-exceptions -fno-rtti

Then I link them with command

gcc -T linker.ld -o myos.bin -ffreestanding -O2 -nostdlib boot.o kernel.o -lgcc -m32

Answer 1

Your bootloader calls a function called kernel_main , but you don't define such a function. Perhaps your main() function should be kernel_main() ?

But, keep in mind that a C++ compiler mangles the symbols (such as function names). So make your main/kernel_main function extern "C", or use objdump or nm on the kernel.o file to figure out the function name that the C++ compilers give those functions, and call that name from your bootloader.

Answer 2

It looks like you need to rename you main() inside kernel.cpp to kernel_main(). The assembly file is attempting to reference and call kernel_main but you have not defined that anywhere in you cpp file. The linker is reporting that error because it cannot find kernel_main(), and it cannot find kernel_main() because it doesn't exist.