linux文件系统与nginx模块系统异曲同工之妙

linux文件系统，通过file_operation结构体，操作inode。file_operation结构体的所有域都是指针结构，每种文件系统都要“实现”这些方法，供系统调用。对于系统来讲，不知道也不用理会现在是需要操作哪种文件系统，它只需调用接口方法，file_operation的函数指针自然会通过初始化过程，调用对应的文件系统的相应方法。

这样看起来，file_operation似乎就是多个接口方法的集合。真正的文件系统只需要实现响应的方法，并且与函数指针对应起来，就可以在初始化之后使用了。

下面是file_operation的结构体代码，其中如果不需要或者不能够使用的函数指针，可以设成NULL。

struct file_operations {
	struct module *owner;
	loff_t (*llseek) (struct file *, loff_t, int);
	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	int (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *, fl_owner_t id);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, struct dentry *, int datasync);
	int (*aio_fsync) (struct kiocb *, int datasync);
	int (*fasync) (int, struct file *, int);
	int (*lock) (struct file *, int, struct file_lock *);
	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
	int (*check_flags)(int);
	int (*flock) (struct file *, int, struct file_lock *);
	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
	int (*setlease)(struct file *, long, struct file_lock **);
};

与之类似的还有inode_operation等结构体。

之后看看nginx的模块系统，为了降低模块和nginx之间的耦合性，并且提高系统的可塑性。nginx也使用了这种面向对象的“接口类”的方法，来实现模块系统。

typedef struct {
    ngx_str_t              *name;

    void                 *(*create_conf)(ngx_cycle_t *cycle);
    char                 *(*init_conf)(ngx_cycle_t *cycle, void *conf);

    ngx_event_actions_t     actions;
} ngx_event_module_t;

typedef struct {
    ngx_int_t  (*add)(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags);
    ngx_int_t  (*del)(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags);

    ngx_int_t  (*enable)(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags);
    ngx_int_t  (*disable)(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags);

    ngx_int_t  (*add_conn)(ngx_connection_t *c);
    ngx_int_t  (*del_conn)(ngx_connection_t *c, ngx_uint_t flags);

    ngx_int_t  (*process_changes)(ngx_cycle_t *cycle, ngx_uint_t nowait);
    ngx_int_t  (*process_events)(ngx_cycle_t *cycle, ngx_msec_t timer,
                   ngx_uint_t flags);

    ngx_int_t  (*init)(ngx_cycle_t *cycle, ngx_msec_t timer);
    void       (*done)(ngx_cycle_t *cycle);
} ngx_event_actions_t;

看，他们是多么的相似。只要实现了event_action结构体内包含的接口方法，就可以实现自己的模块功能。