Name
msgget — get a message queue identifier
Synopsis
int
msgget( |
key_t |
key, |
| |
int |
msgflg); |
DESCRIPTION
The msgget() system call
returns the message queue identifier associated with the
value of the key
argument. A new message queue is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no message queue with the
given key key exists,
and IPC_CREAT is specified in
msgflg.
If msgflg
specifies both IPC_CREAT and
IPC_EXCL and a message queue
already exists for key, then msgget() fails with errno set to EEXIST. (This is analogous to the effect
of the combination O_CREAT |
O_EXCL for open(2).)
Upon creation, the least significant bits of the argument
msgflg define the
permissions of the message queue. These permission bits have
the same format and semantics as the permissions specified
for the mode
argument of open(2). (The execute
permissions are not used.)
If a new message queue is created, then its associated
data structure msqid_ds (see msgctl(2)) is initialised
as follows:
-
msg_perm.cuid and
msg_perm.uid
are set to the effective user ID of the calling
process.
-
msg_perm.cgid and
msg_perm.gid
are set to the effective group ID of the calling
process.
-
The least significant 9 bits of msg_perm.mode are set
to the least significant 9 bits of msgflg.
-
msg_qnum,
msg_lspid,
msg_lrpid,
msg_stime and
msg_rtime are
set to 0.
-
msg_ctime
is set to the current time.
-
msg_qbytes
is set to the system limit MSGMNB.
If the message queue already exists the permissions are
verified, and a check is made to see if it is marked for
destruction.
RETURN VALUE
If successful, the return value will be the message queue
identifier (a non-negative integer), otherwise −1 with
errno indicating the error.
ERRORS
On failure, errno is set to
one of the following values:
- EACCES
-
A message queue exists for key, but the calling
process does not have permission to access the queue,
and does not have the CAP_IPC_OWNER capability.
- EEXIST
-
A message queue exists for key and msgflg specified both
IPC_CREAT and
IPC_EXCL.
- ENOENT
-
No message queue exists for key and msgflg did not specify
IPC_CREAT.
- ENOMEM
-
A message queue has to be created but the system
does not have enough memory for the new data
structure.
- ENOSPC
-
A message queue has to be created but the system
limit for the maximum number of message queues
(MSGMNI) would be
exceeded.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
IPC_PRIVATE isn't a flag
field but a key_t
type. If this special value is used for key, the system call ignores
everything but the least significant 9 bits of msgflg and creates a new
message queue (on success).
The following is a system limit on message queue resources
affecting a msgget() call:
MSGMNI-
System wide maximum number of message queues: policy
dependent (on Linux, this limit can be read and
modified via /proc/sys/kernel/msgmni).
Linux Notes
Until version 2.3.20 Linux would return EIDRM for a
msgget() on a message queue
scheduled for deletion.
BUGS
The name choice IPC_PRIVATE was perhaps unfortunate,
IPC_NEW would more clearly show its function.
SEE ALSO
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)
Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
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Since the Linux kernel and libraries are constantly changing, this
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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>
Removed EIDRM from errors - that can't happen...
Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
Added notes on capability requirements
Modified, 11 Nov 2004, Michael Kerrisk <mtk-manpages@gmx.net>
Language and formatting clean-ups
Added notes on /proc files
|
