minix文件读写中使用的一个额外但是必须的结构filp表

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。