The msgget() argument returns the message queue identifier associated with key.

A message queue identifier and associated message queue and data structure (see intro(2)) are created for key if one of the following are true:

• key is IPC_PRIVATE.
• key does not already have a message queue identifier associated with it, and (msgflg&IPC_CREAT) is true.

On creation, the data structure associated with the new message queue identifier is initialized as follows:

• msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set to the effective user ID and effective group ID, respectively, of the calling process.
• The low-order 9 bits of msg_perm.mode are set to the low-order 9 bits 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.

Upon successful completion, a non-negative integer representing a message queue identifier is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The msgget() function will fail if:

EACCES

A message queue identifier exists for key, but operation permission (see intro(2)) as specified by the low-order 9 bits of msgflg would not be granted.

EEXIST

A message queue identifier exists for key but (msgflg&IPC_CREAT) and (msgflg&IPC_EXCL) are both true.

ENOENT

A message queue identifier does not exist for key and (msgflg&IPC_CREAT) is false.

ENOSPC

A message queue identifier is to be created but the system-imposed limit on the maximum number of allowed message queue identifiers system wide would be exceeded.

intro(2), msgctl(2), msgrcv(2), msgsnd(2), ftok(3C)