为什么以及何时应该使用IPC_NEW来创建信号量？

Question

 static unsigned int state = 1 ;
 key_t h=ftok(".", state++);
 int sem_id=semget(h, no_of_sems, IPC_CREAT|0666);

和
2。

 int sem_id =semget(IPC_NEW,no_of_sems,0666|IPC_CREAT);

正如Linux手册http://man7.org/Linux/man-pages/man2/shmget.2.html中提到的那样

IPC_PRIVATE不是标志字段，而是key_t类型。如果将此特殊值用于key，则系统调用将忽略shmflg中除最低有效9位以外的所有位，并创建一个新的共享内存段。

Edit:
IPC_PRIVATE => IPC_NEW 

Answer 1

首先，你应该观察BUG

   The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would
   more clearly show its function.

好的。我只是解释了我对shmget的了解

请参见shmget的说明

   int shmget(key_t key, size_t size, int shmflg);

DESCRIPTION
   shmget()  returns  the identifier of the System V shared memory segment
   associated with the value of the argument key. A  new  shared  memory
   segment,  with size equal to the value of size rounded up to a multiple
   of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key  isn't
   IPC_PRIVATE,  no shared memory segment corresponding to key exists, and
   IPC_CREAT is specified in shmflg.

 /* resource get request flags */
 #define IPC_CREAT  00001000   /* create if key is nonexistent */
 #define IPC_EXCL   00002000   /* fail if key exists */
 #define IPC_NOWAIT 00004000   /* return error on wait */

   o  The key argument is equal to IPC_PRIVATE.

   o  The key argument does not already have a shared  memory
      identifier  associated  with it, and (shmflg&IPC_CREAT)
      is true.