UNPv2第三章：System V IPC

#include <sys/ipc.h>
key_t ftok( const char * fname, int id )
返回：成功时为IPC键，出错为-1

struct ipc_perm {
   key_t key;  // IPC key
   uid_t uid;      //owner uid
   gid_t gid;      //owner gid
   uid_t cuid; //creator uid
   gid_t cgid; //creator gid
   unsigned short mode;    //read write permission
   unsigned short seq;     //slot usage sequence number
};