zboot/xtract.c

/*
* 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);
}

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zboot/xtract.c

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