/*
* linux/zBoot/xtract.c
*
*
Copyright (C) 1993 Hannu Savolainen
*
*
Extracts the system image and writes it to the
stdout.
* based on tools/build.c by Linus
Torvalds
*/
#include <stdio.h> /*
fprintf */
#include <string.h>
#include <stdlib.h>
/* contains exit */
#include <sys/types.h> /*
unistd.h needs this */
#include <sys/stat.h>
#include
<sys/sysmacros.h>
#include <unistd.h> /*
contains read/write */
#include <fcntl.h>
#include
<a.out.h>
#include <linux/config.h>
#define GCC_HEADER
1024
#define STRINGIFY(x) #x
//中断退出
void die(char *
str)
{
fprintf(stderr,"%s\n",str);
exit(1);
}
//使用方法
void usage(void)
{
die("Usage: xtract system [ | gzip | piggyback >
piggy.s]");
}
//主函数
int main(int argc, char **
argv)
{
int i,c,id, sz;
char
buf[1024];
char major_root, minor_root;
struct stat sb;
//可执行文件指向缓冲区
struct exec *ex = (struct exec *)buf;
//参数一定是两个，否则提示使用方法
if (argc !=
2)
usage();
//打开第二个参数指定的文件
if
((id=open(argv[1],O_RDONLY,0))<0)
die("Unable to open ‘system‘");
//读取GCC文件头
if (read(id,buf,GCC_HEADER) !=
GCC_HEADER)
die("Unable to read header
of ‘system‘");
//校验
if (N_MAGIC(*ex)
!= ZMAGIC)
die("Non-GCC header of
‘system‘");
//计算系统长度
sz =
N_SYMOFF(*ex) - GCC_HEADER + 4; /* +4 to get the same result
than tools/build */
//输出系统长度
fprintf(stderr, "System size is %d\n", sz);
//遍历System文件
while (sz)
{
int l, n;
l = sz;
//如果剩余长度大于buff长度，那么取buff长度
if (l
> sizeof(buf)) l = sizeof(buf);
//读取一定长度
if ((n=read(id, buf, l))
!=l)
{
if (n == -1)
perror(argv[1]);
else
fprintf(stderr, "Unexpected EOF\n");
die("Can‘t read
system");
}
//将内容写入到标准输出中
write(1, buf, l);
sz -=
l;
}
close(id);
return(0);
}
原文地址：http://www.cnblogs.com/xiaofengwei/p/3753349.html
