man-pages/man2/msgget.2

.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
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.\" 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>
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.TH MSGGET 2 2004-05-27 "Linux 2.6.6" "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
with the value of the
.I key
argument.
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
.IR key
exists, 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
If a new message queue is created,
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
On failure,
.B errno
is set to one of the following values:
.TP 11
.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.
.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 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
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
.B msgget
on a message queue scheduled for deletion.
.SH "SEE ALSO"
.BR msgctl (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR ftok (3),
.BR ipc (5),
.BR capabilities (7)