msgget — get a System V message queue identifier
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
int
msgget( |
key_t key, |
int msgflg); |
The msgget() system call
returns the System V 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 initialized
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.
If successful, the return value will be the message queue
identifier (a nonnegative integer), otherwise −1 with
errno indicating the error.
On failure, errno is set to
one of the following values:
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.
IPC_CREAT and
IPC_EXCL were specified
in msgflg, but
a message queue already exists for key.
No message queue exists for key and msgflg did not specify
IPC_CREAT.
A message queue has to be created but the system does not have enough memory for the new data structure.
A message queue has to be created but the system
limit for the maximum number of message queues
(MSGMNI) would be
exceeded.
The inclusion of <sys/types.h> and <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.
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:
MSGMNISystem-wide limit on the number of message queues.
Before Linux 3.19, 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
/proc/sys/kernel/msgmni.
The name choice IPC_PRIVATE
was perhaps unfortunate, IPC_NEW would more clearly show its
function.
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright 1993 Giorgio Ciucci <giorgiocrcc.it> %%%LICENSE_START(VERBATIM) 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. %%%LICENSE_END Added correction due to Nick Duffek <nsdbbc.com>, aeb, 960426 Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond <esrthyrsus.com> Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpagesgmail.com> Removed EIDRM from errors - that can't happen... Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> Language and formatting clean-ups Added notes on /proc files |