ftok原型如下：
key_t ftok( char * fname, int id )
fname就時你指定的文件名，id是子序號。
失敗返回值為-1
ftok應用范圍:在IPC通信及消息記者中使用
解析:
系統建立IPC通訊（如消息隊列、共享內存時）必須指定一個ID值。通常情況下，該id值通過ftok函數得到。
在一般的UNIX實現中，是將文件的索引節點號取出，前面加上子序號得到key_t的返回值。
如指定文件的索引節點號為65538，換算成16進制為0x010002，而你指定的ID值為38，換算成16進制為0x26，則最后的key_t返回值為0x26010002。
查詢文件索引節點號可通過命令： ls -i來查看
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.