msgget - get a message queue identifier
The function returns the message queue identifier associated to the value of the key argument. A new message queue is created if key has value IPC_PRIVATE or key isn't IPC_PRIVATE, no existing message queue is associated to key, and IPC_CREAT is asserted in msgflg (i.e. msgflgIPC_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.)
Furthermore, while creating, 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 MSGMNB.
If the message queue already exists the access permissions are verified, and a check is made to see if it is marked for destruction.
If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error.
For a failing return, errno will be set to one among the following values:
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 success).
Use of IPC_PRIVATE does not actually prohibit other processes from getting access to the allocated message queue.
As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a message queue. Asserting both IPC_CREAT and IPC_EXCL in msgflg only ensures (on success) that a new message queue will be created, it doesn't imply exclusive access to the message queue.
SVr4, SVID. SVr4 does not document the EIDRM error code.