内核编译但不会与unistd.h参考链接

Question

I'm writing a small kernel that has keyboard/cursor support. Everything works great until I add a function from unistd.h

kernel.c

#include "keyboard_map.h"
#include "keyboard.c"
#include "video.c"
#include <unistd.h>

void kmain(void)
{
    terminal_initialize();
    terminal_writestring("Hello, kernel World!\n");
    idt_init();
    kb_init();
    kprint_newline();
    writeSector();
    sleep(1);
    unsigned char c = readSector2();
    kprint((const char *)c);

    while(1);
}

kernel.asm

bits 32

%define     COLS    80

section .text

    ;multiboot spec

    align 4

    dd 0x1BADB002              ;magic

    dd 0x00                    ;flags

    dd - (0x1BADB002 + 0x00)   ;checksum. m+f+c should be zero




global start

global keyboard_handler

global read_port

global write_port

global load_idt


extern kmain        ;this is defined in the c file

extern keyboard_handler_main



read_port:

mov edx, [esp + 4]

        ;al is the lower 8 bits of eax

in al, dx   ;dx is the lower 16 bits of edx

ret




write_port:

mov   edx, [esp + 4]

mov   al, [esp + 4 + 4]

out   dx, al

ret




load_idt:

mov edx, [esp + 4]

lidt [edx]

sti                 ;turn on interrupts

ret




keyboard_handler:                 

call    keyboard_handler_main

iretd




start:

cli                 ;block interrupts

mov esp, stack_space

call kmain

hlt                 ;halt the CPU




section .bss

resb 8192; 8KB for stack

stack_space:

link.ld

OUTPUT_FORMAT(elf32-i386)

ENTRY(start)

SECTIONS

 {

   . = 0x100000;

   .text : { *(.text) }

   .data : { *(.data) }

   .bss  : { *(.bss)  }

 }

I'm compiling it via terminal with a gcc cross compiler, and running it on qemu

nasm -f elf /Users/par/Desktop/kernel.asm -o kasm.o
/usr/local/gcc-4.8.1-for-linux32/bin/i586-pc-linux-gcc -m32 -c -std=c99     /Users/par/Documents/HelloC/HelloC/kernel.c -o kc.o
/usr/local/gcc-4.8.1-for-linux32/bin/i586-pc-linux-ld -m elf_i386 -T /Users/par/Desktop/link.ld -o kernel kasm.o kc.o
qemu-system-i386 -kernel kernel

I'm new to c programming so hopefully this is an easy question for someone out there. There are several other c files in the project, but I'm confident the problem has got to be somewhere above. Again everything works if the unistd.h and sleep() are removed, but will create an error that says "Undefined reference to function sleep() in kc.o" when trying to link with that reference.

Also these references work fine elsewhere in the project...

 #include <stddef.h> 
 #include <stdint.h>  

...which further confuses me why unistd.h won't link... but will compile.

Answer 1

Including unistd.h just tells the compiler about the things (functions, etc.) defined in a library. It describes the interface to the library but not the implementation of the libraries functionality.

That's why your program compiles. unistd.h has a declaration of sleep() in it. It probably looks something like this:

unsigned sleep(unsigned);

You can look at it, it's a text file.

The problem is that when you try to link, nothing defines sleep(). Normally sleep() and other standard functions exist in a precompiled library. Something with a name like libc.a The .a file is an archive containing object files that implement the library's functionality which the linker can link with your object files to resolve references to things like sleep().

The reason that stddef.h stdint.h work is that they primarily define types and macros that are used at compile time but don't need any additional object code to be lined in at link time.

Your link line doesn't include any libraries. I suspect that's because you don't have any for your bare-metal environment.

You have a couple of options. You could try to port a preexisting library to work in your environment, or you can write your own sleep() function.

You can take a look at ELLCC which is one (of several) cross development tool chains that provide libraries. ELLCC's bare-metal libraries are a work in progress, but looking at the ELK blog posts might give you some ideas.