ipc 通讯之ftok

man ftok 记录如下：

NAME
       ftok - convert a pathname and a project identifier to a System V IPC key


SYNOPSIS
       # include <sys/types.h>
       # include <sys/ipc.h>


       key_t ftok(const char *pathname, int proj_id);


DESCRIPTION
       The  ftok()  function  uses the identity of the file named by the given pathname (which must refer to an existing, accessible file) and
       the least significant 8 bits of proj_id (which must be non-zero) to generate a key_t type System V  IPC  key,  suitable  for  use  with
       msgget(2), semget(2), or shmget(2).


       The  resulting  value is the same for all pathnames that name the same file, when the same value of proj_id is used. The value returned
       should be different when the (simultaneously existing) files or the project IDs differ.


RETURN VALUE
       On success the generated key_t value is returned. On failure -1 is returned, with errno indicating the error as for the stat(2)  system
       call.


CONFORMING TO
       POSIX.1-2001.


NOTES
       Under libc4 and libc5 (and under SunOS 4.x) the prototype was
              key_t ftok(char *pathname, char proj_id);
       Today  proj_id  is  an  int, but still only 8 bits are used. Typical usage has an ASCII character proj_id, that is why the behaviour is
       said to be undefined when proj_id is zero.


       Of course no guarantee can be given that the resulting key_t is unique. Typically, a best effort attempt  combines  the  given  proj_id
       byte,  the  lower  16 bits of the i-node number, and the lower 8 bits of the device number into a 32-bit result.  Collisions may easily
       happen, for example between files on /dev/hda1 and files on /dev/sda1.

说明：

pathname就时你指定的文件名(该文件必须是存在而且可以访问的)，id是子序号，虽然为int，但是只有8个比特被使用(0-255)。
当成功执行的时候，一个key_t值将会被返回，否则 -1 被返回。
 在一般的UNIX实现中，是将文件的索引节点号取出，前面加上子序号得到key_t的返回值。如指定文件的索引节点号为65538，换算成16进制为 0x010002，而你指定的ID值为38，换算成16进制为0x26，则最后的key_t返回值为0x26010002。
查询文件索引节点号的方法是： ls -i