ftok原型如下：
key_t ftok( char * fname, int id )
fname就时你指定的文件名，id是子序号。
失败返回值为-1
ftok应用范围:在IPC通信及消息记者中使用
解析:
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.
CONFORMING TO
XPG4
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.