get message queue
returns the message queue identifier associated with
key. A message queue identifier is a unique integer
greater than zero.
A message queue is created if either key is
IPC_PRIVATE, or key
does not have a message queue identifier associated with it, and the
IPC_CREAT bit is set in
If a new message queue is created, the data structure associated with it (the msqid_ds structure, see msgctl(2)) is initialized as follows:
- msg_perm.cuid and msg_perm.uid are set to the effective UID of the calling process.
- msg_perm.gid and msg_perm.cgid are set to the effective GID of the calling process.
- msg_perm.mode is set to the lower 9 bits of msgflg.
- msg_cbytes, msg_qnum, msg_lspid, msg_lrpid, msg_rtime, and msg_stime are set to 0.
- msg_qbytes is set to the system wide
maximum value for the number of bytes in a queue
- msg_ctime is set to the current time.
Upon successful completion a positive message queue identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.
- A message queue is already associated with key and the caller has no permission to access it.
IPC_EXCLare set in msgflg, and a message queue is already associated with key.
- A new message queue could not be created because the system limit for the number of message queues has been reached.
IPC_CREATwas not set in msgflg and no message queue associated with key was found.
msgctl(2), msgrcv(2), msgsnd(2), ftok(3)
Message queues appeared in AT&T System V Release 1 UNIX.