.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)

.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>

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.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>

.\" Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>

.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added notes on CAP_IPC_OWNER requirement

.\" Modified, 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID

.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"	Language and formatting clean-ups

.\"	Added msqid_ds and ipc_perm structure definitions

.\" 2005-08-02, mtk: Added IPC_INFO, MSG_INFO, MSG_STAT descriptions

.\"

.TH MSGCTL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

msgctl \- System V message control operations

.SH SYNOPSIS

.nf

.B #include <sys/types.h>

.B #include <sys/ipc.h>

.B #include <sys/msg.h>

.PP

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

.fi

.SH DESCRIPTION

.BR msgctl ()

performs the control operation specified by

.I cmd

on the System\ V message queue with identifier

.IR msqid .

.PP

The

.I msqid_ds

data structure is defined in \fI<sys/msg.h>\fP as follows:

.PP

.in +4n

.EX

struct msqid_ds {

    struct ipc_perm msg_perm;     /* Ownership and permissions */

    time_t          msg_stime;    /* Time of last msgsnd(2) */

    time_t          msg_rtime;    /* Time of last msgrcv(2) */

    time_t          msg_ctime;    /* Time of last change */

    unsigned long   __msg_cbytes; /* Current number of bytes in

                                     queue (nonstandard) */

    msgqnum_t       msg_qnum;     /* Current number of messages

                                     in queue */

    msglen_t        msg_qbytes;   /* Maximum number of bytes

                                     allowed in queue */

    pid_t           msg_lspid;    /* PID of last msgsnd(2) */

    pid_t           msg_lrpid;    /* PID of last msgrcv(2) */

};

.EE

.in

.PP

The

.I ipc_perm

structure is defined as follows

(the highlighted fields are settable using

.BR IPC_SET ):

.PP

.in +4n

.EX

struct ipc_perm {

    key_t          __key;       /* Key supplied to msgget(2) */

    uid_t          \fBuid\fP;         /* Effective UID of owner */

    gid_t          \fBgid\fP;         /* Effective GID of owner */

    uid_t          cuid;        /* Effective UID of creator */

    gid_t          cgid;        /* Effective GID of creator */

    unsigned short \fBmode\fP;        /* Permissions */

    unsigned short __seq;       /* Sequence number */

};

.EE

.in

.PP

Valid values for

.I cmd

are:

.TP

.B IPC_STAT

Copy information from the kernel data structure associated with

.I msqid

into the

.I msqid_ds

structure pointed to by

.IR buf .

The caller must have read permission on the message queue.

.TP

.B IPC_SET

Write the values of some members of the

.I msqid_ds

structure pointed to by

.I buf

to the kernel data structure associated with this message queue,

updating also its

.I msg_ctime

member.

The following members of the structure are updated:

.IR msg_qbytes ,

.IR msg_perm.uid ,

.IR msg_perm.gid ,

and (the least significant 9 bits of)

.IR msg_perm.mode .

The effective UID of the calling process must match the owner

.RI ( msg_perm.uid )

or creator

.RI ( msg_perm.cuid )

of the message queue, or the caller must be privileged.

Appropriate privilege (Linux: the

.B CAP_SYS_RESOURCE

capability) is required to raise the

.I msg_qbytes

value beyond the system parameter

.BR MSGMNB .

.TP

.B IPC_RMID

Immediately remove the message queue,

awakening all waiting reader and writer processes (with an error

return and

.I errno

set to

.BR EIDRM ).

The calling process must have appropriate privileges

or its effective user ID must be either that of the creator or owner

of the message queue.

The third argument to

.BR msgctl ()

is ignored in this case.

.TP

.BR IPC_INFO " (Linux-specific)"

Return information about system-wide message queue limits and

parameters in the structure pointed to by

.IR buf .

This structure is of type

.I msginfo

(thus, a cast is required),

defined in

.I <sys/msg.h>

if the

.B _GNU_SOURCE

feature test macro is defined:

.IP

.in +4n

.EX

struct msginfo {

    int msgpool; /* Size in kibibytes of buffer pool

                    used to hold message data;

                    unused within kernel */

    int msgmap;  /* Maximum number of entries in message

                    map; unused within kernel */

    int msgmax;  /* Maximum number of bytes that can be

                    written in a single message */

    int msgmnb;  /* Maximum number of bytes that can be

                    written to queue; used to initialize

                    msg_qbytes during queue creation

                    (msgget(2)) */

    int msgmni;  /* Maximum number of message queues */

    int msgssz;  /* Message segment size;

                    unused within kernel */

    int msgtql;  /* Maximum number of messages on all queues

                    in system; unused within kernel */

    unsigned short int msgseg;

                 /* Maximum number of segments;

                    unused within kernel */

};

.EE

.in

.IP

The

.IR msgmni ,

.IR msgmax ,

and

.I msgmnb

settings can be changed via

.I /proc

files of the same name; see

.BR proc (5)

for details.

.TP

.BR MSG_INFO " (Linux-specific)"

Return a

.I msginfo

structure containing the same information as for

.BR IPC_INFO ,

except that the following fields are returned with information

about system resources consumed by message queues: the

.I msgpool

field returns the number of message queues that currently exist

on the system; the

.I msgmap

field returns the total number of messages in all queues

on the system; and the

.I msgtql

field returns the total number of bytes in all messages

in all queues on the system.

.TP

.BR MSG_STAT " (Linux-specific)"

Return a

.I msqid_ds

structure as for

.BR IPC_STAT .

However, the

.I msqid

argument is not a queue identifier, but instead an index into

the kernel's internal array that maintains information about

all message queues on the system.

.SH RETURN VALUE

On success,

.BR IPC_STAT ,

.BR IPC_SET ,

and

.B IPC_RMID

return 0.

A successful

.B IPC_INFO

or

.B MSG_INFO

operation returns the index of the highest used entry in the

kernel's internal array recording information about all

message queues.

(This information can be used with repeated

.B MSG_STAT

operations to obtain information about all queues on the system.)

A successful

.B MSG_STAT

operation returns the identifier of the queue whose index was given in

.IR msqid .

.PP

On error, \-1 is returned with

.I errno

indicating the error.

.SH ERRORS

On failure,

.I errno

is set to one of the following:

.TP

.B EACCES

The argument

.I cmd

is equal to

.B IPC_STAT

or

.BR MSG_STAT ,

but the calling process does not have read permission on the message queue

.IR msqid ,

and does not have the

.B CAP_IPC_OWNER

capability in the user namespace that governs its IPC namespace.

.TP

.B EFAULT

The argument

.I cmd

has the value

.B IPC_SET

or

.BR IPC_STAT ,

but the address pointed to by

.I buf

isn't accessible.

.TP

.B EIDRM

The message queue was removed.

.TP

.B EINVAL

Invalid value for

.I cmd

or

.IR msqid .

Or: for a

.B MSG_STAT

operation, the index value specified in

.I msqid

referred to an array slot that is currently unused.

.TP

.B EPERM

The argument

.I cmd

has the value

.B IPC_SET

or

.BR IPC_RMID ,

but the effective user ID of the calling process is not the creator

(as found in

.IR msg_perm.cuid )

or the owner

(as found in

.IR msg_perm.uid )

of the message queue,

and the caller is not privileged (Linux: does not have the

.B CAP_SYS_ADMIN

capability).

.TP

.B EPERM

An attempt

.RB ( IPC_SET )

was made to increase

.I msg_qbytes

beyond the system parameter

.BR MSGMNB ,

but the caller is not privileged (Linux: does not have the

.B CAP_SYS_RESOURCE

capability).

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

.\" SVID does not document the EIDRM error condition.

.SH NOTES

The inclusion of

.I <sys/types.h>

and

.I <sys/ipc.h>

isn't required on Linux or by any version of POSIX.

However,

some old implementations required the inclusion of these header files,

and the SVID also documented their inclusion.

Applications intended to be portable to such old systems may need

to include these header files.

.\" Like Linux, the FreeBSD man pages still document

.\" the inclusion of these header files.

.PP

The

.BR IPC_INFO ,

.B MSG_STAT

and

.B MSG_INFO

operations are used by the

.BR ipcs (1)

program to provide information on allocated resources.

In the future these may modified or moved to a

.I /proc

filesystem interface.

.PP

Various fields in the \fIstruct msqid_ds\fP were

typed as

.I short

under Linux 2.2

and have become

.I long

under Linux 2.4.

To take advantage of this,

a recompilation under glibc-2.1.91 or later should suffice.

(The kernel distinguishes old and new calls by an

.B IPC_64

flag in

.IR cmd .)

.SH SEE ALSO

.BR msgget (2),

.BR msgrcv (2),

.BR msgsnd (2),

.BR capabilities (7),

.BR mq_overview (7),

.BR svipc (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.