1.为什么需要他

因为子进程与父进程之间需要共享文件读写指针。

2.构成

代码：

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++                                      servers/fs/file.h++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 21700    /* This is the filp table.  It is an intermediary between file descriptors and 21701     * inodes.  A slot is free if filp_count == 0. 21702     */ 21703     21704    EXTERN struct filp { 21705      mode_t filp_mode;             /* RW bits, telling how file is opened */ 21706      int filp_flags;               /* flags from open and fcntl */ 21707      int filp_count;               /* how many file descriptors share this slot?*/ 21708      struct inode *filp_ino;       /* pointer to the inode */ 21709      off_t filp_pos;               /* file position */   File: Page: 928 servers/fs/file.h 21710     21711      /* the following fields are for select() and are owned by the generic 21712       * select() code (i.e., fd-type-specific select() code can't touch these). 21713       */ 21714      int filp_selectors;           /* select()ing processes blocking on this fd */ 21715      int filp_select_ops;          /* interested in these SEL_* operations */ 21716     21717      /* following are for fd-type-specific select() */ 21718      int filp_pipe_select_ops; 21719    } filp[NR_FILPS]; 21720     21721    #define FILP_CLOSED     0       /* filp_mode:  associated device closed */ 21722     21723    #define NIL_FILP (struct filp *) 0      /* indicates absence of a filp slot */

filp真正必须包含的内容是共享文件的位置，但是实现时候将inode也放在里面。

这么做的好处是，在进程表中的文件描述符数组只需要包含指向filp表项的指针。