.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
-.\" and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
+.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" 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@gmx.net>
+.\" 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@gmx.net>
+.\" 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@gmx.net>
+.\" 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 2004-11-10 "Linux 2.6.9" "Linux Programmer's Manual"
+.TH MSGCTL 2 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
-msgctl \- message control operations
+msgctl \- System V message control operations
.SH SYNOPSIS
.nf
-.B
-#include <sys/types.h>
-.B
-#include <sys/ipc.h>
-.B
-#include <sys/msg.h>
+.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
-.sp
-.BI "int msgctl(int " msqid ,
-.BI "int " cmd ,
-.BI "struct msqid_ds *" buf );
.SH DESCRIPTION
.BR msgctl ()
performs the control operation specified by
.I cmd
-on the message queue with identifier
+on the System\ V message queue with identifier
.IR msqid .
.PP
The
.I msqid_ds
-data structure is defined in <sys/msg.h> as follows:
-.nf
+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() */
- time_t msg_rtime; /* Time of last msgrcv() */
- time_t msg_ctime; /* Time of last change */
- unsigned long __msg_cbytes; /* Current number of bytes in
- queue (non-standard) */
- 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() */
- pid_t msg_lrpid; /* PID of last msgrcv() */
+ 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; /* Creation time/time of last
+ modification via msgctl() */
+ 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) */
};
-.in -4n
-.fi
+.EE
+.in
+.PP
+The fields of the
+.I msgid_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_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.
+.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 ()
+.BR IPC_SET
+operation.
.PP
The
.I ipc_perm
-structure is defined in <sys/ipc.h> as follows
+structure is defined as follows
(the highlighted fields are settable using
.BR IPC_SET ):
.PP
-.nf
.in +4n
+.EX
struct ipc_perm {
- key_t key; /* Key supplied to msgget() */
- 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 */
+ 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 */
};
-.in -4n
-.fi
+.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
.RI ( msg_perm.cuid )
of the message queue, or the caller must be privileged.
Appropriate privilege (Linux: the
-.B CAP_IPC_RESOURCE
+.B CAP_SYS_RESOURCE
capability) is required to raise the
.I msg_qbytes
value beyond the system parameter
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)"
-Returns information about system-wide message queue limits and
+.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
-.IR msginfo
+.I msginfo
(thus, a cast is required),
defined in
.I <sys/msg.h>
-if the _GNU_SOURCE feature test macro is defined:
-.nf
-.in +2n
-
+if the
+.B _GNU_SOURCE
+feature test macro is defined:
+.IP
+.in +4n
+.EX
struct msginfo {
- int msgpool; /* Size in bytes of buffer pool used
- to hold message data; unused */
- int msgmap; /* Max. # of entries in message
- map; unused */
- int msgmax; /* Max. # of bytes that can be
+ 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; /* Max. # of bytes that can be written to
- queue; used to initialize msg_qbytes
- during queue creation (msgget()) */
- int msgmni; /* Max. # of message queues */
- int msgssz; /* Message segment size; unused */
- int msgtql; /* Max. # of messages on all queues
- in system; unused */
+ 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;
- /* Max. # of segments; unused */
+ /* Maximum number of segments;
+ unused within kernel */
};
-
-.in -2n
-.fi
+.EE
+.in
+.IP
The
.IR msgmni ,
.IR msgmax ,
.BR proc (5)
for details.
.TP
-.BR MSG_INFO " (Linux specific)"
-Returns a
+.BR MSG_INFO " (Linux-specific)"
+Return a
.I msginfo
structure containing the same information as for
.BR IPC_INFO ,
field returns the total number of bytes in all messages
in all queues on the system.
.TP
-.BR MSG_STAT " (Linux specific)"
-Returns a
+.BR MSG_STAT " (Linux-specific)"
+Return a
.I msqid_ds
structure as for
.BR IPC_STAT .
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"
+.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
+.IR msqid
+meaning that any user can employ this operation (just as any user may read
+.IR /proc/sysvipc/msg
+to obtain the same information).
+.SH RETURN VALUE
On success,
.BR IPC_STAT ,
.BR IPC_SET ,
and
-.BR IPC_RMID
+.B IPC_RMID
return 0.
A successful
.B IPC_INFO
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 error, \-1 is returned with
.I errno
indicating the error.
On failure,
.I errno
is set to one of the following:
-.TP 11
+.TP
.B EACCES
The argument
.I cmd
is equal to
-.BR IPC_STAT
+.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.
+capability in the user namespace that governs its IPC namespace.
.TP
.B EFAULT
The argument
(as found in
.IR msg_perm.uid )
of the message queue,
-and the process is not privileged (Linux: it does not have the
+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 ,
-.BR MSG_STAT
+.B MSG_STAT
and
.B MSG_INFO
operations are used by the
-.BR ipcs (8)
+.BR ipcs (1)
program to provide information on allocated resources.
-In the future these may modified or moved to a /proc file system
-interface.
-
-Various fields in the \fIstruct msqid_ds\fP were shorts under Linux 2.2
-and have become longs under Linux 2.4. To take advantage of this,
+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 IPC_64 flag in
+(The kernel distinguishes old and new calls by an
+.B IPC_64
+flag in
.IR cmd .)
-.SH "CONFORMING TO"
-SVr4, POSIX.1-2001.
-.\" SVID does not document the EIDRM error condition.
-.SH "SEE ALSO"
+.SH SEE ALSO
.BR msgget (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR capabilities (7),
.BR mq_overview (7),
-.BR svipc (7)
+.BR sysvipc (7)
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