linux文件系统总结

apue中：其中进程表项内部的数组又称为 进程打开文件表   

另外一个角度：

从linux内核角度开：

task_struct是进程描述符对应上面的进程表项，在task_struct描述符中有struct file_struct*类型的变量file，指向struct file_struct结构。

1.file_struct:该结构体有进程描述符中的files域指向，如下：

struct files_struct {

        atomic_t    count;              /* structure's usage count */

        spinlock_t  file_lock;          /* lock protecting this structure */

        int         max_fds;            /* maximum number of file objects */

        int         max_fdset;          /* maximum number of file descriptors */

        int         next_fd;            /* next file descriptor number */

        struct file **fd;               /* array of all file objects */

        fd_set      *close_on_exec;     /* file descriptors to close on exec() */

        fd_set      *open_fds;           /* pointer to open file descriptors */

        fd_set      close_on_exec_init; /* initial files to close on exec() */

        fd_set      open_fds_init;      /* initial set of file descriptors */

        struct file *fd_array[NR_OPEN_DEFAULT]; /* default array of file objects */

};

该结构中有一个struct file *fd_array[NR_OPEN_DEFAULT]的数组，对应上面的进程打开文件表，其数组的元素指向struct file 结构，此结构对应apue中的文件表
2、文件对象:文件对象表示进程以打开的文件。文件对象仅仅在进程观点上代表已打开文件，它反过来指向目录项对象(反过来指向索引节点)，其实只有目录项对象才表示已打开的实际文件。虽然一个文件对应的文件对象不是唯一的，但对应的索引节点和目录项对象无疑是唯一的。文件对象由file结构表示，定义在文件linux/fs.h中，如下:

struct file {

        struct list_head       f_list;        /* list of file objects */

        struct dentry          *f_dentry;     /* associated dentry object */

        struct vfsmount        *f_vfsmnt;     /* associated mounted fs */

        struct file_operations *f_op;         /* file operations table */

        atomic_t               f_count;       /* file object's usage count */

        unsigned int           f_flags;       /* flags specified on open */

        mode_t                 f_mode;        /* file access mode */

        loff_t                 f_pos;         /* file offset (file pointer) */

        struct fown_struct     f_owner;       /* owner data for signals */

        unsigned int           f_uid;         /* user's UID */

        unsigned int           f_gid;         /* user's GID */

        int                    f_error;       /* error code */

        struct file_ra_state   f_ra;          /* read-ahead state */

        unsigned long          f_version;     /* version number */

        void                   *f_security;   /* security module */

        void                   *private_data; /* tty driver hook */

        struct list_head       f_ep_links;    /* list of eventpoll links */

        spinlock_t             f_ep_lock;     /* eventpoll lock */

        struct address_space   *f_mapping;    /* page cache mapping */

};

文件表项中包括了文件描述符的状态标志，文件的偏移量（或者说文件的位置），和v节点指针（unix独有）

struct file 包括我们通常说的文件描述符引用计数，文件偏移量，文件模式等等以系列核心功能，同时又指向文件file_operation结构的指针

struct file_operations {

        struct module *owner;

        loff_t (*llseek) (struct file *, loff_t, int);

        ssize_t (*read) (struct file *, char *, size_t, loff_t *);

        ssize_t (*aio_read) (struct kiocb *, char *, size_t, loff_t);

        ssize_t (*write) (struct file *, const char *, size_t, loff_t *);

        ssize_t (*aio_write) (struct kiocb *, const char *, size_t, 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);

        int (*mmap) (struct file *, struct vm_area_struct *);

        int (*open) (struct inode *, struct file *);

        int (*flush) (struct file *);

        int (*release) (struct inode *, struct file *);

        int (*fsync) (struct file *, struct dentry *, int);

        int (*aio_fsync) (struct kiocb *, int);

        int (*fasync) (int, struct file *, int);

        int (*lock) (struct file *, int, struct file_lock *);

        ssize_t (*readv) (struct file *, const struct iovec *,

                          unsigned long, loff_t *);

        ssize_t (*writev) (struct file *, const struct iovec *,

                           unsigned long, loff_t *);

        ssize_t (*sendfile) (struct file *, loff_t *, size_t,

                             read_actor_t, void *);

        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 flags);

        int (*dir_notify) (struct file *filp, unsigned long arg);

        int (*flock) (struct file *filp, int cmd, struct file_lock *fl);

};

struct file包括指向struct dentry 的指针

struct dentry {

        atomic_t                 d_count;      /* usage count */

        unsigned long            d_vfs_flags;  /* dentry cache flags */

        spinlock_t               d_lock;       /* per-dentry lock */

        struct inode             *d_inode;     /* associated inode */

        struct list_head         d_lru;        /* unused list */

        struct list_head         d_child;      /* list of dentries within */

        struct list_head         d_subdirs;    /* subdirectories */

        struct list_head         d_alias;      /* list of alias inodes */

        unsigned long            d_time;       /* revalidate time */

        struct dentry_operations *d_op;        /* dentry operations table */

        struct super_block       *d_sb;        /* superblock of file */

        unsigned int             d_flags;      /* dentry flags */

        int                      d_mounted;    /* is this a mount point? */

        void                     *d_fsdata;    /* filesystem-specific data */

        struct rcu_head          d_rcu;        /* RCU locking */

        struct dcookie_struct    *d_cookie;    /* cookie */

        struct dentry            *d_parent;    /* dentry object of parent */

        struct qstr              d_name;       /* dentry name */

        struct hlist_node        d_hash;       /* list of hash table entries */

        struct hlist_head        *d_bucket;    /* hash bucket */

        unsigned char            d_iname[DNAME_INLINE_LEN_MIN]; /* short name */

};

由于目录项并非真正保存在磁盘上，所有目录项没有对应的磁盘数据结构，VFS根据字符串形式的路径名现场创建它，目录项结构体也没有是否被修改的标志。目录项对象有三种状态：被使用，未被使用和负状态。一个被使用的目录项对应一个有效的索引节点(即d_inode指向相应的索引节点)并且该对象存在一个或多个使用者(即d_count为正值)。一个未被使用的目录项对应一个有效的索引节点(d_inode指向一个索引节点)，但是VFS当前并未使用它(d_count为0)。该目录项对象仍然指向一个有效对象，而且被保留在内存中以便需要时再使用它。显然这样要比重新创建要效率高些。一个负状态的目录项没有对应的有效索引节点(d_inode为NULL).因为索引节点已被删除了，或路径不再正确了，但是目录项仍然保留，以便快速解析以后的路径查询。虽然负的状态目录项有些用处，但如果需要的话话，还是可以删除的，可以销毁它。

只是路径对应的innode节点