NAME

msgctl - message control operations

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

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

DESCRIPTION

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.

msgctl fails if one or more of the following are true:

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.

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

EPERM cmd is IPC_RMID or IPC_SET. The effective user ID of the calling process is not that of 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.

SEE ALSO

intro(2), msgget(2), msgop(2).

DIAGNOSTICS

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.