写操作系统----7.实现printf

1.1 整理一下代码

cong@msi:/work/os/code/6print$ tree
.
├── boot
│   ├── loader.S
│   └── mbr.S
├── include
│   ├── print.h
│   └── stdint.h
├── kernel
│   └── main.c
├── lib
│   ├── printf.c
│   ├── printf.o
│   ├── put_char.o
│   └── put_char.S
├── Makefile
├── makefile_bak
└── Makefile_bak

1.2 Makefile

cong@msi:/work/os/code/6print$ cat Makefile
ENTRY_POINT = 0xc0001500
AS = nasm
CC = gcc
LD = ld
LIB = -I lib/ -I lib/kernel/ -I lib/user/ -I kernel/ -I device/
ASFLAGS = -f elf
CFLAGS = -Wall $(LIB) -c -fno-builtin -W -Wstrict-prototypes \
         -Wmissing-prototypes
LDFLAGS = -Ttext $(ENTRY_POINT) -e main -Map $(BUILD_DIR)/kernel.map

all: mbr loader kern
mbr:
    nasm ./boot/mbr.S -I ./boot/include/ -o ./boot/mbr.bin
loader:
    nasm ./boot/loader.S -I ./boot/include/ -o ./boot/loader.bin
kern:put_char.o printf.o main.o
    ld -m elf_i386 -Ttext 0xc0001500 -e main -o ./kernel/kernel.bin \
        ./kernel/main.o ./lib/put_char.o ./lib/printf.o
main.o:
    gcc $(CFLAGS) -c -m32 -I ./include -o ./kernel/main.o ./kernel/main.c
printf.o:
    gcc $(CFLAGS) -c -m32 -I ./include -o ./lib/printf.o ./lib/printf.c
put_char.o:
    nasm $(ASFLAGS) ./lib/put_char.S -o ./lib/put_char.o

clean:
    -rm /work/os/code/disk.img ./boot/mbr.bin ./boot/loader.bin ./kernel/*.o ./kernel/kernel.bin
flash:
    -rm /work/os/code/disk.img
    dd if=/dev/zero of=/work/os/code/disk.img bs=1M count=30
    dd if=./boot/mbr.bin of=/work/os/code/disk.img bs=512 count=1 conv=notrunc
    dd if=./boot/loader.bin of=/work/os/code/disk.img bs=512 count=5 seek=2 conv=notrunc
    dd if=./kernel/kernel.bin of=/work/os/code/disk.img bs=512 count=200 seek=9 conv=notrunc
show:
    echo "show loader.bin"
    xxd ./loader.bin
    echo "show disk.img"
    xxd -seek 0x400 -l 0x200 /work/os/code/disk.img

1.3 printf.c
原书中实现了put_char，那么进一步就可以实现printf

cong@msi:/work/os/code/6print/lib$ cat printf.c
#include "print.h"
#include <stdarg.h>
void put_string(char *str)
{
    while(*str!='\0')
    {
        put_char(*str);
        str++;
    }
}
static void itoa(char **buf, unsigned char i, unsigned char base)    -->这儿有个bug，unsigned char应该换成int
{
    static char *s;
#define LEN 20
    unsigned char rem;
    static char rev[LEN+1];

    rev[LEN] = 0;
    if (i == 0)
    {
        (*buf)[0] = '0';
        ++(*buf);
        return;
    }
    s = &rev[LEN];
    while (i)
    {
        rem = i % base;
        if (rem < 10)
            *--s = rem + '0';
        else if (base == 16)
            *--s = "abcdef"[rem - 10];
        i /= base;
    }
    while (*s)
    {
        (*buf)[0] = *s++;
        ++(*buf);
    }
}
char print_buf[250];
void printf(char *fmt,...)
{
    va_list ap;
    int ival;
    char *p;
    char *bp;
    bp= print_buf;
    *bp= '\0';

    va_start (ap, fmt);
    for (p= fmt; *p; p++)
    {
        if (*p != '%')
        {
            *bp++= *p;
            continue;
        }
        switch (*++p) {
            case 'd':
                ival= va_arg(ap, int);
                itoa (&bp, ival, 10);
                break;
            case 'x':
                ival= va_arg(ap, int);
                itoa (&bp, ival, 16);
                break;
        }
    }
    *bp = '\0';
    put_string(print_buf);
    va_end (ap);
}

1.4 main函数测试

cong@msi:/work/os/code/6print$ cat kernel/main.c
#include "print.h"
void main(void)
{
    int a=3;
    put_char('k');
    printf("a=%d\n",a);
    while(1)
    {
    }
    return ;
}

1.5 代码打包
6print.rar (下载后改名为6print.tar.gz)
1.6 运行测试


printf实现了

1.7  一个问题
1.7.1 printf.c中使用了系统中的stdarg.h
Makefile中加入-nostdinc时就会报错
./lib/printf.c:2:20: fatal error: stdarg.h: No such file or directory

1.7.2 自己实现一个va_list

#ifndef __LIB_STDARG_H
#define __LIB_STDARG_H
typedef char * va_list;
#define _INTSIZEOF(n) ((sizeof(n)+sizeof(int)-1)&~(sizeof(int) - 1) )
#define va_start(ap,v) ( ap = (va_list)&v + _INTSIZEOF(v) )
#define va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define va_end(ap) ( ap = (va_list)0 )
#endif

1.7.3 代码打包
6print.rar (下载后改名为6print.tar.gz)