ftok原型如下：
                        key_t ftok( char * fname, int id )
                        fname就时你指定的文件(存在,可以访问的)名，id是子序号。
先来简单的叙述一下,如果不太清楚的话,再接着向下看:
                ftok函数是根据pathname和id来创建一个关键字(类型为 key_t)，此关键字在创建信号量，创建消息队列的时候都需要使用。其中pathname必须是一个存在的可访问的路径或文件，id必须不得为0。

失败返回值为-1

ftok应用范围

         英文是这样描述的; generate  a  key_t  type  System V IPC key, suitable for use with msgget(2),semget(2), or shmget(2).

系统建立IPC通讯（如消息队列、共享内存时）必须指定一个ID值。通常情况下，该id值通过ftok函数得到。

在一般的UNIX实现中，是将pathname指定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可通过命令： ls -i    来查看

    nux Programmer's Manual                   FTOK(3)
                                                                                                                                      
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 nonzero) 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.