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.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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
.\" 2018-03-20, dbueso: Added MSG_STAT_ANY description.
.\"
.TH MSGCTL 2 2021-03-22 "Linux man-pages (unreleased)"
.SH NAME
msgctl \- System V message control operations
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.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 creation or last
                                   modification by msgctl() */
    unsigned long   msg_cbytes; /* # of bytes in queue */
    msgqnum_t       msg_qnum;   /* # number of messages in queue */
    msglen_t        msg_qbytes; /* Maximum # of bytes in queue */
    pid_t           msg_lspid;  /* PID of last msgsnd(2) */
    pid_t           msg_lrpid;  /* PID of last msgrcv(2) */
};
.EE
.in
.PP
The fields of the
.I msqid_ds
structure are as follows:
.TP 11
.I msg_perm
This is an
.I ipc_perm
structure (see below) that specifies the access permissions on the message
queue.
.TP
.I msg_stime
Time of the last
.BR msgsnd (2)
system call.
.TP
.I msg_rtime
Time of the last
.BR msgrcv (2)
system call.
.TP
.I msg_ctime
Time of creation of queue or time of last
.BR msgctl ()
.B IPC_SET
operation.
.TP
.I msg_cbytes
Number of bytes in all messages currently on the message queue.
This is a nonstandard Linux extension that is not specified in POSIX.
.TP
.I msg_qnum
Number of messages currently on the message queue.
.TP
.I msg_qbytes
Maximum number of bytes of message text allowed on the message
queue.
.TP
.I msg_lspid
ID of the process that performed the last
.BR msgsnd (2)
system call.
.TP
.I msg_lrpid
ID of the process that performed the last
.BR msgrcv (2)
system call.
.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
The least significant 9 bits of the
.I mode
field of the
.I ipc_perm
structure define the access permissions for the message queue.
The permission bits are as follows:
.TS
l l.
0400	Read by user
0200	Write by user
0040	Read by group
0020	Write by group
0004	Read by others
0002	Write by others
.TE
.PP
Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
.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.
.IP
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 .
.IP
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 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.
.TP
.BR MSG_STAT_ANY " (Linux-specific, since Linux 4.17)"
Return a
.I msqid_ds
structure as for
.BR MSG_STAT .
However,
.I msg_perm.mode
is not checked for read access for
.I msqid
meaning that any user can employ this operation (just as any user may read
.I /proc/sysvipc/msg
to obtain the same information).
.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
or
.B MSG_STAT_ANY
operations to obtain information about all queues on the system.)
A successful
.B MSG_STAT
or
.B MSG_STAT_ANY
operation returns the identifier of the queue whose index was given in
.IR msqid .
.PP
On failure, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.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 STANDARDS
POSIX.1-2001, POSIX.1-2008, SVr4.
.\" SVID does not document the EIDRM error condition.
.SH NOTES
The
.BR IPC_INFO ,
.BR 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 sysvipc (7)