1.首先打开我们通过dd命令生成的image文件
fd = open(argv[1], O_RDWR);
2.写超级块
write_superblock(fd)
我们详细看看超级块是怎么去写的
static int write_superblock(int fd)
{
struct simplefs_super_block sb = {
.version = 1,
.magic = SIMPLEFS_MAGIC,
.block_size = SIMPLEFS_DEFAULT_BLOCK_SIZE,
/* One inode for rootdirectory and another for a welcome file that we are going to create */
.inodes_count = 2,
/* FIXME: Free blocks management is not implemented yet */
.free_blocks = (~0) & ~(1 << WELCOMEFILE_DATABLOCK_NUMBER),
};
ssize_t ret;
ret = write(fd, &sb, sizeof(sb));
if (ret != SIMPLEFS_DEFAULT_BLOCK_SIZE) {
printf
("bytes written [%d] are not equal to the default block size\n",
(int)ret);
return -1;
}
printf("Super block written succesfully\n");
return 0;
}
Look,很简单,直接向块设备写入simplefs_super_block 结构的数据即可,当前的超级块的版本为1,魔数,每个数据块的大小为4K,inode的个数为2,当前空闲的数据块。
3.写入根节点
static int write_inode_store(int fd)
{
ssize_t ret;
struct simplefs_inode root_inode;
root_inode.mode = S_IFDIR;
root_inode.inode_no = SIMPLEFS_ROOTDIR_INODE_NUMBER;
root_inode.data_block_number = SIMPLEFS_ROOTDIR_DATABLOCK_NUMBER;
root_inode.dir_children_count = 1;
ret = write(fd, &root_inode, sizeof(root_inode));
if (ret != sizeof(root_inode)) {
printf
("The inode store was not written properly. Retry your mkfs\n");
return -1;
}
printf("root directory inode written succesfully\n");
return 0;
}
紧跟在super block后面的就是root inode,这个inode是一个目录,在linux中dentry本身也是一个特殊的inode,我们将这个特殊的inode的inode号设置为了1,同时这个inode存放在data block的bitmap中的第二个。另外在根denty的下面还有一个文件。
4.在根dentry下面的这个文件是,我们现在对这个文件进行写入
struct simplefs_inode welcome = {
.mode = S_IFREG,
.inode_no = WELCOMEFILE_INODE_NUMBER,
.data_block_number = WELCOMEFILE_DATABLOCK_NUMBER,
.file_size = sizeof(welcomefile_body),
};
write_inode(fd, &welcome)
write_inode的实现如下:
static int write_inode(int fd, const struct simplefs_inode *i)
{
off_t nbytes;
ssize_t ret;
ret = write(fd, i, sizeof(*i));
if (ret != sizeof(*i)) {
printf
("The welcomefile inode was not written properly. Retry your mkfs\n");
return -1;
}
printf("welcomefile inode written succesfully\n");
nbytes = SIMPLEFS_DEFAULT_BLOCK_SIZE - sizeof(*i) - sizeof(*i);
ret = lseek(fd, nbytes, SEEK_CUR);
if (ret == (off_t)-1) {
printf
("The padding bytes are not written properly. Retry your mkfs\n");
return -1;
}
printf
("inode store padding bytes (after the two inodes) written sucessfully\n");
return 0;
}
上述代码首先写入了一个inode,然后对齐4K的边界;
5.写dentry里面的内容
struct simplefs_dir_record record = {
.filename = "vanakkam",
.inode_no = WELCOMEFILE_INODE_NUMBER,
};
write_dirent(fd, &record)
也就是说在根dentry的下面记录着如下信息
文件名:“vanakkam”
对应的inode号是:2
也就是上面第4步对应的普通文件
6.对这个普通文件写入内容
char welcomefile_body[] = “Love is God. God is Love. Anbe Murugan.\n”;