你需要了解linux中的mknod

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);
}

include\linux\types.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\n", 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