The msgctl() function provides a variety of message control operations as specified by cmd. The following cmds are available:

IPC_STAT

Place the current value of each member of the data structure associated with msqid into the structure pointed to by buf. The contents of this structure are defined in intro(2).

IPC_SET

Set the value of the following members of the data structure associated with msqid to the corresponding value found in the structure pointed to by buf:

msg_perm.uid msg_perm.gid msg_perm.mode /* access permission bits only */ msg_qbytes

This cmd can only be executed by a process that has an effective user ID equal to either that of super-user, or to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid. Only super-user can raise the value of msg_qbytes.

IPC_RMID

Remove the message queue identifier specified by msqid from the system and destroy the message queue and data structure associated with it. This cmd can only be executed by a process that has an effective user ID equal to either that of super-user, or to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid. The buf argument is ignored.

Upon successful completion, msgctl() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.

The msgctl() function will fail if:

EACCES

The cmd argument is IPC_STAT and operation permission is denied to the calling process (see intro(2)).

EFAULT

The buf argument points to an illegal address.

EINVAL

The msqid argument is not a valid message queue identifier; or the cmd argument is not a valid command or is IPC_SET and msg_perm.uid or msg_perm.gid is not valid.

EPERM

The cmd argument is IPC_RMID or IPC_SET and the effective user ID of the calling process is not super-user and is not equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.

EPERM

The cmd argument is IPC_SET, an attempt is being made to increase to the value of msg_qbytes, and the effective user ID of the calling process is not super-user.

EOVERFLOW

The cmd argument is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf.

intro(2), msgget(2), msgrcv(2), msgsnd(2)