The function returns the message queue identifier associated with the value of
the key argument. A new message queue is created if key has the
value IPC_PRIVATE or key isn't IPC_PRIVATE, no message
queue with the given key key exists, and IPC_CREAT is asserted
in msgflg (i.e., msgflg&IPC_CREAT is nonzero).
The presence in msgflg of the fields IPC_CREAT and
IPC_EXCL plays the same role, with respect to the existence of the
message queue, as the presence of O_CREAT and O_EXCL in the mode
argument of the open(2) system call: i.e. the msgget function
fails if msgflg asserts both IPC_CREAT and IPC_EXCL and a
message queue already exists for key.
Upon creation, the lower 9 bits of the argument msgflg define the access
permissions of the message queue. These permission bits have the same format
and semantics as the access permissions parameter in open(2) or
creat(2) system calls. (The execute permissions are not used.)
If a new message queue is created, the system call initializes the system
message queue data structure msqid_ds as follows:
msg_perm.cuid and msg_perm.uid are set to the
effective user-ID of the calling process.
msg_perm.cgid and msg_perm.gid are set to the
effective group-ID of the calling process.
The lowest order 9 bits of msg_perm.mode are set to
the lowest order 9 bit of msgflg.
msg_qnum, msg_lspid, msg_lrpid,
msg_stime and msg_rtime are set to 0.
msg_ctime is set to the current time.
msg_qbytes is set to the system limit
If the message queue already exists the access permissions are verified, and a
check is made to see if it is marked for destruction.
IPC_PRIVATE isn't a flag field but a key_t type. If this special
value is used for key, the system call ignores everything but the
lowest order 9 bits of msgflg and creates a new message queue (on
The following is a system limit on message queue resources affecting a
System wide maximum number of message queues: policy