一、源码分析：
mknod->系统调用->
/fs/namei.c
SYSCALL_DEFINE3(mknod, const char __user *, filename, int, mode, unsigned, dev)
{
return sys_mknodat(AT_FDCWD, filename, mode, dev);
}
SYSCALL_DEFINE4(mknodat, int, dfd, const char __user *, filename, int, mode, unsigned, dev)
{
int error;
char *tmp;
struct dentry *dentry;
struct nameidata nd;
...
error = user_path_parent(dfd, filename, &nd, &tmp);
...
dentry = lookup_create(&nd, 0);
...
if (!IS_POSIXACL(nd.path.dentry->d_inode)) mode &= ~current_umask();
error = may_mknod(mode);
...
error = mnt_want_write(nd.path.mnt);
...
error = security_path_mknod(&nd.path, dentry, mode, dev);
...
switch (mode & S_IFMT)
{
case 0: case S_IFREG:
error = vfs_create(nd.path.dentry->d_inode,dentry,mode,&nd);
break;
case S_IFCHR: case S_IFBLK:
error = vfs_mknod(nd.path.dentry->d_inode, dentry, mode, new_decode_dev(dev));
break;
case S_IFIFO: case S_IFSOCK:
error = vfs_mknod(nd.path.dentry->d_inode,dentry,mode,0);
break;
}
...
}
使用new_decode_dev(dev)创建设备号
static inline dev_t new_decode_dev(u32 dev)
{
unsigned major = (dev & 0xfff00) >> 8;
unsigned minor = (dev & 0xff) | ((dev >> 12) & 0xfff00);
return MKDEV(major, minor);
}
includelinux ypes.h
typedef __kernel_dev_t dev_t;
typedef __u32 __kernel_dev_t;
vfs_create()：用于普通文件创建
vfs_mkdir()：用于目录节点创建
vfs_mknod()：用于特殊文件（FIFO、插口、字符设备文件、块设备文件）创建
/proc目录下的特殊文件，则由内核生成，非用户创建。
int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
{
int error = may_create(dir, dentry);
...
if ((S_ISCHR(mode) || S_ISBLK(mode)) && !ns_capable(inode_userns(dir), CAP_MKNOD)) return -EPERM;
...
error = devcgroup_inode_mknod(mode, dev);
error = security_inode_mknod(dir, dentry, mode, dev);
error = dir->i_op->mknod(dir, dentry, mode, dev);
...
}
dir ： innode结构指针，指向待创建设备文件的父节点，由path_walk()找到的。
dentry：指向代表着或将要代表待创建设备文件节点的目录项dentry结构，sys_mknod中lookup_create在内核dentry结构杂凑表中找到或创建
vfs_mknod是虚拟文件系统的mknod
下面还有诸如ext2、ext3、ext4、jffs、yaffs等实际文件系统，对应struct inode_operations jfs_dir_inode_operations
中的.mknod = jfs_mknod,
static int jfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t rdev)
{
...
struct inode *ip;
...
ip = ialloc(dir, mode);
...
init_special_inode(ip, ip->i_mode, rdev);
...
}
ialloc分配inode结构体，init_special_inode进行填充
/fs/inode.c
void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
{
inode->i_mode = mode;
if (S_ISCHR(mode)) //字符设备
{
inode->i_fop = &def_chr_fops;
inode->i_rdev = rdev;
}
else if (S_ISBLK(mode)) //块设备
{
inode->i_fop = &def_blk_fops;
inode->i_rdev = rdev;
}
else if (S_ISFIFO(mode))//FIFO流设备
inode->i_fop = &def_fifo_fops;
else if (S_ISSOCK(mode))//网络设备
inode->i_fop = &bad_sock_fops;
else//未知设备提示
printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o) for inode %s:%lu
", mode, inode->i_sb->s_id,inode->i_ino);
}
EXPORT_SYMBOL(init_special_inode);
函数设置file_operation的i_fop和设备号i_rdev