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tools/build.c
/*
* linux/tools/build.c
*
* Copyright (C) 1991, 1992 Linus Torvalds
*/
/*
* This file builds a disk-image from three different files:
*
* - bootsect: max 510 bytes of 8086 machine code, loads the rest
* - setup: max 4 sectors of 8086 machine code, sets up system parm
* - system: 80386 code for actual system
*
* It does some checking that all files are of the correct type, and
* just writes the result to stdout, removing headers and padding to
* the right amount. It also writes some system data to stderr.
*/
/*
* Changes by tytso to allow root device specification
*/
#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 <linux/config.h>
#include <linux/a.out.h>
#define MINIX_HEADER 32
#define GCC_HEADER 1024
#define SYS_SIZE DEF_SYSSIZE
#define DEFAULT_MAJOR_ROOT 0
#define DEFAULT_MINOR_ROOT 0
/* max nr of sectors of setup: don‘t change unless you also change
* bootsect etc */
#define SETUP_SECTS 4
#define STRINGIFY(x) #x
//联合
typedef union {
long l;
short s[2];
char b[4];
} conv;
//定义long数据类型
long intel_long(long l)
{
conv t;
t.b[0] = l & 0xff; l >>= 8;
t.b[1] = l & 0xff; l >>= 8;
t.b[2] = l & 0xff; l >>= 8;
t.b[3] = l & 0xff; l >>= 8;
return t.l;
}
//定义short类型
short intel_short(short l)
{
conv t;
t.b[0] = l & 0xff; l >>= 8;
t.b[1] = l & 0xff; l >>= 8;
return t.s[0];
}
//程序异常结束
void die(char * str)
{
fprintf(stderr,"%s\n",str);
exit(1);
}
//用法，build程序将bootsect setup system三部分和根设备名生成映像文件
void usage(void)
{
die("Usage: build bootsect setup system [rootdev] [> image]");
}
//主函数
int main(int argc, char ** argv)
{
int i,c,id, sz;
//系统长度
unsigned long sys_size;
//定义缓冲区长度
char buf[1024];
//定义可执行文件结构指针，将缓冲区转化为可执行结构
struct exec *ex = (struct exec *)buf;
//主从根设备
char major_root, minor_root;
//文件状态
struct stat sb;
//如果输入参数个数不是4或者5，则显示用法信息
if ((argc < 4) || (argc > 5))
usage();
//如果参数个数是5个，即带有根设备文件名
if (argc > 4) {
//此时输入参数是5个，比较第五个参数是否为CURRENT
if (!strcmp(argv[4], "CURRENT")) {
if (stat("/", &sb)) {
perror("/");
die("Couldn‘t stat /");
}
major_root = major(sb.st_dev);
minor_root = minor(sb.st_dev);
} else if (strcmp(argv[4], "FLOPPY")) {//比较第五个参数是否为FLOPPY，即软盘
if (stat(argv[4], &sb)) {
perror(argv[4]);
die("Couldn‘t stat root device.");
}
major_root = major(sb.st_rdev);
minor_root = minor(sb.st_rdev);
} else {
major_root = 0;
minor_root = 0;
}
} else {//输入参数是四个，即没有指定根设备，则此处使用默认的根设备
major_root = DEFAULT_MAJOR_ROOT;
minor_root = DEFAULT_MINOR_ROOT;
}
//输出主从根设备号
fprintf(stderr, "Root device is (%d, %d)\n", major_root, minor_root);
//初始化buff
for (i=0;i<sizeof buf; i++) buf[i]=0;
//打开bootsect，如果出错，则中断程序
if ((id=open(argv[1],O_RDONLY,0))<0)
die("Unable to open ‘boot‘");
//读取文件头，且该文件头应该是MINIX文件头，长度为32，如果出错，则中断程序
if (read(id,buf,MINIX_HEADER) != MINIX_HEADER)
die("Unable to read header of ‘boot‘");
//前四个字节应该为0x04100301
if (((long *) buf)[0]!=intel_long(0x04100301))
die("Non-Minix header of ‘boot‘");
//判断头部长度是否为32，四个字节中后面三个为0
if (((long *) buf)[1]!=intel_long(MINIX_HEADER))
die("Non-Minix header of ‘boot‘");
//数据段长度是否为0
if (((long *) buf)[3] != 0)
die("Illegal data segment in ‘boot‘");
//bss段是否为0
if (((long *) buf)[4] != 0)
die("Illegal bss in ‘boot‘");
//判断执行点处是否为0
if (((long *) buf)[5] != 0)
die("Non-Minix header of ‘boot‘");
//判断符号表长度是否为0
if (((long *) buf)[7] != 0)
die("Illegal symbol table in ‘boot‘");
//读取数据，返回的长度i应该为512
i=read(id,buf,sizeof buf);
//输出boot sector长度
fprintf(stderr,"Boot sector %d bytes.\n",i);
//如果实际读取的长度不是512，则报错，退出
if (i != 512)
die("Boot block must be exactly 512 bytes");
//最后两字节如果不是启动扇区标志0x55AA则报错，退出
if ((*(unsigned short *)(buf+510)) != (unsigned short)intel_short(0xAA55))
die("Boot block hasn‘t got boot flag (0xAA55)");
//将主从设备号写入引导块
buf[508] = (char) minor_root;
buf[509] = (char) major_root;
//将所读取的512字节写入标准输出
i=write(1,buf,512);
//校验写入数据的长度
if (i!=512)
die("Write call failed");
//关闭文件。完成bootsect文件的处理工作
close (id);
//打开第二个参数指定的文件setup，出错则退出
if ((id=open(argv[2],O_RDONLY,0))<0)
die("Unable to open ‘setup‘");
//读取MINIX文件头，长度应为32
if (read(id,buf,MINIX_HEADER) != MINIX_HEADER)
die("Unable to read header of ‘setup‘");
//校验文件头魔数
if (((long *) buf)[0]!=intel_long(0x04100301))
die("Non-Minix header of ‘setup‘");
//校验文件头长度
if (((long *) buf)[1]!=intel_long(MINIX_HEADER))
die("Non-Minix header of ‘setup‘");
//判断数据段的长度是否为0
if (((long *) buf)[3] != 0)
die("Illegal data segment in ‘setup‘");
//判断bss段的长度是否为0
if (((long *) buf)[4] != 0)
die("Illegal bss in ‘setup‘");
//判断执行点处是否为0
if (((long *) buf)[5] != 0)
die("Non-Minix header of ‘setup‘");
//判断符号表的长度是否为0
if (((long *) buf)[7] != 0)
die("Illegal symbol table in ‘setup‘");
//每次读取1024字节，并将读取的数据写入标准输出
for (i=0 ; (c=read(id,buf,sizeof buf))>0 ; i+=c )
if (write(1,buf,c)!=c)
die("Write call failed");
//最后一次操作长度是否为0
if (c != 0)
die("read-error on ‘setup‘");
//以上都没有问题，则关闭文件
close (id);
//校验setup程序的长度
if (i > SETUP_SECTS*512)
die("Setup exceeds " STRINGIFY(SETUP_SECTS)
" sectors - rewrite build/boot/setup");
//输出setup的长度
fprintf(stderr,"Setup is %d bytes.\n",i);
//缓冲区清零
for (c=0 ; c<sizeof(buf) ; c++)
buf[c] = ‘\0‘;
//如果setup程序长度小于SETUP_SECTS*512
while (i<SETUP_SECTS*512) {
//剩余空间填\0,补足SETUP_SECTS*512
c = SETUP_SECTS*512-i;
if (c > sizeof(buf))
c = sizeof(buf);
if (write(1,buf,c) != c)
die("Write call failed");
i += c;
}
//打开system文件
if ((id=open(argv[3],O_RDONLY,0))<0)
die("Unable to open ‘system‘");
//读取文件头，该文件头为GCC_HEADER
if (read(id,buf,GCC_HEADER) != GCC_HEADER)
die("Unable to read header of ‘system‘");
//此时ex指向该文件头，校验文件头魔数
if (N_MAGIC(*ex) != ZMAGIC)
die("Non-GCC header of ‘system‘");
//输出所system文件长度、代码段长度、数据段长度和bss段长度
fprintf(stderr,"System is %d kB (%d kB code, %d kB data and %d kB bss)\n",
(ex->a_text+ex->a_data+ex->a_bss)/1024,
ex->a_text /1024,
ex->a_data /1024,
ex->a_bss /1024);
//
sz = N_SYMOFF(*ex) - GCC_HEADER + 4;
sys_size = (sz + 15) / 16;
//校验system的长度
if (sys_size > SYS_SIZE)
die("System is too big");
//每次读取1024个字节长度，然后写入标准输出中
//最后一次是实际长度
while (sz > 0) {
int l, n;
l = sz;
if (l > sizeof(buf))
l = sizeof(buf);
if ((n=read(id, buf, l)) != l) {
if (n == -1)
perror(argv[1]);
else
fprintf(stderr, "Unexpected EOF\n");
die("Can‘t read ‘system‘");
}
if (write(1, buf, l) != l)
die("Write failed");
sz -= l;
}
//关闭文件
close(id);
//从标准输出中从开始搜索偏移为500字节处
if (lseek(1,500,0) == 500) {
//如果可以搜索到
//在buff[0]和buff[1]出写入system的长度
buf[0] = (sys_size & 0xff);
buf[1] = ((sys_size >> 8) & 0xff);
if (write(1, buf, 2) != 2)
die("Write failed");
}
return(0);
}
//本文件的主要功能是将编译产生的三个文件bootsect、setup和system是三个文件写入一个映像文件中
//前面两个文件时MINIX文件，后面是一个GCC文件，写入标准输出时需要校验文件格式