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Manual page for msgctl(2)

msgctl - message control operations

#include <sys/msg.h>

int msgctl(int msqid, int cmd, struct msqid_ds *buf );

msgctl() 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 /* only access permission bits */
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. buf 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: cmd is IPC_STAT and operation permission is denied to the calling process (see intro.2
EFAULT: buf points to an illegal address.
EINVAL: msqid is not a valid message queue identifier.
EINVAL: cmd is not a valid command.
EINVAL: cmd is IPC_SET and msg_perm.uid or msg_perm.gid is not valid.
EPERM: cmd is IPC_RMID or IPC_SET. The effective user of the calling process is not super-user, or the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.
EPERM: cmd 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 that of super-user.
EOVERFLOW: cmd is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf.