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.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\"
.\" 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.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" 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>
.TH MSGGET 2 "November 1, 1993" "Linux 0.99.13" "Linux Programmer's Manual"
.SH NAME
msgget \- get a message queue identifier
.SH SYNOPSIS
.nf
.B
# include <sys/types.h>
.B
# include <sys/ipc.h>
.B
# include <sys/msg.h>
.fi
.sp
.BI "int msgget ( key_t " key ,
.BI "int "msgflg " )"
.SH DESCRIPTION
The function returns the message queue identifier associated
to the value of the
.I key
argument.
A new message queue is created if
.I key
has value
.B IPC_PRIVATE
or
.I key
isn't
.BR IPC_PRIVATE ,
no existing message queue is associated to
.IR key ,
and
.B IPC_CREAT
is asserted in
.I msgflg
(i.e.
.IB msgflg & IPC_CREAT
is nonzero).
The presence in
.I msgflg
of the fields
.B IPC_CREAT
and
.B IPC_EXCL
plays the same role, with respect to the existence
of the message queue, as the presence
of
.B O_CREAT
and
.B O_EXCL
in the mode argument of the
.BR open (2)
system call: i.e. the
.B msgget
function fails if
.I msgflg
asserts both
.B IPC_CREAT
and
.B IPC_EXCL
and a message queue already exists for
.IR key .
.PP
Upon creation, the lower 9 bits of the argument
.I msgflg
define the access permissions of the message queue.
These permission bits have the same format and semantics
as the access permissions parameter in
.BR open (2)
or
.BR creat (2)
system calls. (The execute permissions are not used.)
.PP
Furthermore, while creating,
the system call initializes the system message queue data structure
.B msqid_ds
as follows:
.IP
.B msg_perm.cuid
and
.B msg_perm.uid
are set to the effective user\-ID of the calling process.
.IP
.B msg_perm.cgid
and
.B msg_perm.gid
are set to the effective group\-ID of the calling process.
.IP
The lowest order 9 bits of
.B msg_perm.mode
are set to the lowest order 9 bit of
.IR msgflg .
.IP
.BR msg_qnum ,
.BR msg_lspid ,
.BR msg_lrpid ,
.BR msg_stime
and
.B msg_rtime
are set to 0.
.IP
.B msg_ctime
is set to the current time.
.IP
.B msg_qbytes
is set to the system limit
.BR MSGMNB .
.PP
If the message queue already exists the access permissions are
verified, and a check is made to see if it is marked for
destruction.
.SH "RETURN VALUE"
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise
.B \-1
with
.B errno
indicating the error.
.SH ERRORS
For a failing return,
.B errno
will be set to one among the following values:
.TP 11
.B EACCES
A message queue exists for
.IR key ,
but the calling process has no access permissions to the queue.
.TP
.B EEXIST
A message queue exists for
.B key
and
.I msgflg
was asserting both
.B IPC_CREAT
and
.BR IPC_EXCL .
.TP
.B EIDRM
The message queue is marked for removal.
.TP
.B ENOENT
No message queue exists for
.I key
and
.I msgflg
wasn't asserting
.BR IPC_CREAT .
.TP
.B ENOMEM
A message queue has to be created but the system has not 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 NOTES
.B IPC_PRIVATE
isn't a flag field but a
.B key_t
type.
If this special value is used for
.IR key ,
the system call ignores everything but the lowest order 9 bits of
.I msgflg
and creates a new message queue (on success).
.PP
The following is a system limit on message queue resources affecting a
.B msgget
call:
.TP 11
.B MSGMNI
System wide maximum number of message queues: policy
dependent.
.SH BUGS
Use of
.B IPC_PRIVATE
does not actually prohibit other processes from getting
access to the allocated message queue.
.PP
As for the files, there is currently no intrinsic way for a process to ensure
exclusive access to a message queue.
Asserting both
.B IPC_CREAT
and
.B IPC_EXCL
in
.I msgflg
only ensures (on success) that a new message queue will be created,
it doesn't imply exclusive access to the message queue.
.SH "CONFORMING TO"
SVr4, SVID. SVr4 does not document the EIDRM error code.
.SH "SEE ALSO"
.BR ftok (3),
.BR ipc (5),
.BR msgctl (2),
.BR msgsnd (2),
.BR msgrcv (2)
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