MSGCTL(2) — SYSTEM CALLS

NAME

msgctl − message control operations

SYNOPSIS

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

int msgctl (msqid, cmd, buf)
int msqid, 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 low 9 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.

In the msgop(2) and msgctl(2) system call descriptions, the permission required for an operation is given as "[token]", where “token” is the type of permission needed interpreted as follows:

00400 Read by user

00200 Write by user

00060 Read, Write by group

00006 Read, Write by others

Read and Write permissions on a msqid are granted to a process if one or more of the following are true:

The effective user ID of the process is super-user.

The effective user ID of the process matches msg_perm.[c]uid in the data structure associated with msqid and the appropriate bit of the “user” portion (0600) of msg_perm.mode is set.

The effective user ID of the process does not match msg_perm.[c]uid and the effective group ID of the process matches msg_perm.[c]gid and the appropriate bit of the “group” portion (060) of msg_perm.mode is set.

The effective user ID of the process does not match msg_perm.[c]uid and the effective group ID of the process does not match msg_perm.[c]gid and the appropriate bit of the “other” portion (06) of msg_perm.mode is set.

Otherwise, the corresponding permissions are denied.

RETURN VALUES

msgctl() returns:

0 on success.

−1 on failure and sets errno to indicate the error.

ERRORS

EACCES cmd is equal to IPC_STAT and [READ] 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.

cmd is not a valid command.

EPERM cmd is equal to IPC_RMID or IPC_SET. The effective user ID of the calling process is neither super-user, nor the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.

cmd is equal to 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 equal to that of super-user.

Museum