MSGCTL(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
msgctl — XSI message control operations

SYNOPSIS

#include <sys/msg.h>

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

DESCRIPTION
The msgctl() function operates on XSI message queues (see the Base Definitions volume of POSIX.1-2017, Section 3.226, Message Queue). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.

The msgctl() function shall provide message control operations as specified by cmd. The following values for cmd, and the message control operations they specify, are:

IPC_STAT
    Place the current value of each member of the msqid_ds data structure associated with msqid into the structure pointed to by buf. The contents of this structure are defined in <sys/msg.h>.

IPC_SET
    Set the value of the following members of the msqid_ds 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
        msg_qbytes

    Also, the msg_ctime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

    IPC_SET can only be executed by a process with appropriate privileges or that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the msqid_ds data structure associated with msqid. Only a process with appropriate privileges 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 msqid_ds data structure associated with it. IPC_RMD can only be executed by a process with appropriate privileges or one that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the msqid_ds data structure associated with msqid.

RETURN VALUE
Upon successful completion, msgctl() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS
The msgctl() function shall fail if:

EACCES
    The argument cmd is IPC_STAT and the calling process does not have read permission; see Section 2.7, XSI Interprocess Communication.

EINVAL
    The value of msqid is not a valid message queue identifier; or the value of cmd is not a valid command.

EPERM
    The argument cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.

EPERM
    The argument 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 does not have appropriate privileges.

The following sections are informative.

EXAMPLES
None.

APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for interprocess communication (IPC). Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgget(), msgrcv(), msgsnd()

The Base Definitions volume of POSIX.1-2017, Section 3.226, Message Queue, <sys_msg.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

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