.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\"
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.\" Added correction due to Nick Duffek <nsd@bbc.com>, aeb, 960426
.\" Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond <esr@thyrsus.com>
-.\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk-manpages@gmx.net>
+.\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com>
.\" Removed EIDRM from errors - that can't happen...
-.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
+.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Added notes on capability requirements
-.\" 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 notes on /proc files
.\"
-.TH MSGGET 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual"
+.TH MSGGET 2 2019-08-02 "Linux" "Linux Programmer's Manual"
.SH NAME
-msgget \- get a message queue identifier
+msgget \- get a System V message queue identifier
.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 msgget(key_t " key ", int " msgflg );
.fi
-.sp
-.BI "int msgget(key_t " key ,
-.BI "int " msgflg );
.SH DESCRIPTION
The
.BR msgget ()
-system call returns the message queue identifier associated
+system call returns the System\ V message queue identifier associated
with the value of the
.I key
argument.
+It may be used either to obtain the identifier of a previously created
+message queue (when
+.I msgflg
+is zero and
+.I key
+does not have the value
+.BR IPC_PRIVATE ),
+or to create a new set.
+.PP
A new message queue is created if
.I key
has the value
isn't
.BR IPC_PRIVATE ,
no message queue with the given key
-.IR key
+.I key
exists, and
.B IPC_CREAT
is specified in
.IR key ,
then
.BR msgget ()
-fails with
+fails with
.I errno
set to
.BR EEXIST .
-(This is analogous to the effect of the combination
+(This is analogous to the effect of the combination
.B O_CREAT | O_EXCL
for
.BR open (2).)
.I msgflg
define the permissions of the message queue.
These permission bits have the same format and semantics
-as the permissions specified for the
+as the permissions specified for the
.I mode
argument of
.BR open (2).
.I msqid_ds
(see
.BR msgctl (2))
-is initialised as follows:
+is initialized as follows:
.IP
.I msg_perm.cuid
and
.IR msg_qnum ,
.IR msg_lspid ,
.IR msg_lrpid ,
-.IR msg_stime
+.IR msg_stime ,
and
.I msg_rtime
are set to 0.
If the message queue already exists the permissions are
verified, and a check is made to see if it is marked for
destruction.
-.SH "RETURN VALUE"
+.SH RETURN VALUE
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise \-1
with
On failure,
.I errno
is set to one of the following values:
-.TP 11
+.TP
.B EACCES
A message queue exists for
.IR key ,
but the calling process does not have permission to access the queue,
and does not have the
-.BR CAP_IPC_OWNER
-capability.
+.B CAP_IPC_OWNER
+capability in the user namespace that governs its IPC namespace.
.TP
.B EEXIST
-A message queue exists for
-.I key
-and
-.I msgflg
-specified both
.B IPC_CREAT
and
-.BR IPC_EXCL .
+.BR IPC_EXCL
+were specified in
+.IR msgflg ,
+but a message queue already exists for
+.IR key .
.TP
.B ENOENT
No message queue exists for
number of message queues
.RB ( MSGMNI )
would be exceeded.
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008, SVr4.
.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
.B IPC_PRIVATE
isn't a flag field but a
.I key_t
The following is a system limit on message queue resources affecting a
.BR msgget ()
call:
-.TP 11
+.TP
.B MSGMNI
-System wide maximum number of message queues: policy
-dependent
-(on Linux, this limit can be read and modified via
-.IR /proc/sys/kernel/msgmni ).
-.SH BUGS
-The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
-would more clearly show its function.
-.SH "CONFORMING TO"
-SVr4, SVID.
-Until version 2.3.20 Linux would return EIDRM for a
+System-wide limit on the number of message queues.
+Before Linux 3.19,
+.\" commit 0050ee059f7fc86b1df2527aaa14ed5dc72f9973
+the default value for this limit was calculated using a formula
+based on available system memory.
+Since Linux 3.19, the default value is 32,000.
+On Linux, this limit can be read and modified via
+.IR /proc/sys/kernel/msgmni .
+.SS Linux notes
+Until version 2.3.20, Linux would return
+.B EIDRM
+for a
.BR msgget ()
on a message queue scheduled for deletion.
-.SH "SEE ALSO"
+.SH BUGS
+The name choice
+.B IPC_PRIVATE
+was perhaps unfortunate,
+.B IPC_NEW
+would more clearly show its function.
+.SH SEE ALSO
.BR msgctl (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR ftok (3),
-.BR ipc (5),
.BR capabilities (7),
-.BR mq_overview (7)
+.BR mq_overview (7),
+.BR sysvipc (7)
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