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.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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@gmail.com>
.\" Removed EIDRM from errors - that can't happen...
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Added notes on capability requirements
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Language and formatting clean-ups
.\" Added notes on /proc files
.\"
.TH msgget 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
msgget \- get a System V message queue identifier
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/msg.h>
.P
.BI "int msgget(key_t " key ", int " msgflg );
.fi
.SH DESCRIPTION
The
.BR msgget ()
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.
.P
A new message queue is created if
.I key
has the value
.B IPC_PRIVATE
or
.I key
isn't
.BR IPC_PRIVATE ,
no message queue with the given key
.I key
exists, and
.B IPC_CREAT
is specified in
.IR msgflg .
.P
If
.I msgflg
specifies both
.B IPC_CREAT
and
.B IPC_EXCL
and a message queue already exists for
.IR key ,
then
.BR msgget ()
fails with
.I errno
set to
.BR EEXIST .
(This is analogous to the effect of the combination
.B O_CREAT | O_EXCL
for
.BR open (2).)
.P
Upon creation, the least significant bits of the argument
.I msgflg
define the permissions of the message queue.
These permission bits have the same format and semantics
as the permissions specified for the
.I mode
argument of
.BR open (2).
(The execute permissions are not used.)
.P
If a new message queue is created,
then its associated data structure
.I msqid_ds
(see
.BR msgctl (2))
is initialized as follows:
.IP \[bu] 3
.I msg_perm.cuid
and
.I msg_perm.uid
are set to the effective user ID of the calling process.
.IP \[bu]
.I msg_perm.cgid
and
.I msg_perm.gid
are set to the effective group ID of the calling process.
.IP \[bu]
The least significant 9 bits of
.I msg_perm.mode
are set to the least significant 9 bits of
.IR msgflg .
.IP \[bu]
.IR msg_qnum ,
.IR msg_lspid ,
.IR msg_lrpid ,
.IR msg_stime ,
and
.I msg_rtime
are set to 0.
.IP \[bu]
.I msg_ctime
is set to the current time.
.IP \[bu]
.I msg_qbytes
is set to the system limit
.BR MSGMNB .
.P
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
On success,
.BR msgget ()
returns the message queue identifier (a nonnegative integer).
On failure, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.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
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EEXIST
.B IPC_CREAT
and
.B IPC_EXCL
were specified in
.IR msgflg ,
but a message queue already exists for
.IR key .
.TP
.B ENOENT
No message queue exists for
.I key
and
.I msgflg
did not specify
.BR IPC_CREAT .
.TP
.B ENOMEM
A message queue has to be created but the system does not have enough
memory for the new data structure.
.TP
.B ENOSPC
A message queue has to be created but the system limit for the maximum
number of message queues
.RB ( MSGMNI )
would be exceeded.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.SS Linux
Until Linux 2.3.20, Linux would return
.B EIDRM
for a
.BR msgget ()
on a message queue scheduled for deletion.
.SH NOTES
.B IPC_PRIVATE
isn't a flag field but a
.I key_t
type.
If this special value is used for
.IR key ,
the system call ignores everything but the least significant 9 bits of
.I msgflg
and creates a new message queue (on success).
.P
The following is a system limit on message queue resources affecting a
.BR msgget ()
call:
.TP
.B MSGMNI
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 .
.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 capabilities (7),
.BR mq_overview (7),
.BR sysvipc (7)
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