mq_overview.7: Update details on defaults and ceiling for 'msgsize_max' limit

[thirdparty/man-pages.git] / man7 / mq_overview.7

'\" t
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%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.\" manual page may be incorrect or out-of-date.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
.\"
.TH MQ_OVERVIEW 7 2014-09-21 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_overview \- overview of POSIX message queues
.SH DESCRIPTION
POSIX message queues allow processes to exchange data in
the form of messages.
This API is distinct from that provided by System V message queues
.RB ( msgget (2),
.BR msgsnd (2),
.BR msgrcv (2),
etc.), but provides similar functionality.

Message queues are created and opened using
.BR mq_open (3);
this function returns a
.I message queue descriptor
.RI ( mqd_t ),
which is used to refer to the open message queue in later calls.
Each message queue is identified by a name of the form
.IR /somename ;
that is, a null-terminated string of up to
.BI NAME_MAX
(i.e., 255) characters consisting of an initial slash,
followed by one or more characters, none of which are slashes.
Two processes can operate on the same queue by passing the same name to
.BR mq_open (3).

Messages are transferred to and from a queue using
.BR mq_send (3)
and
.BR mq_receive (3).
When a process has finished using the queue, it closes it using
.BR mq_close (3),
and when the queue is no longer required, it can be deleted using
.BR mq_unlink (3).
Queue attributes can be retrieved and (in some cases) modified using
.BR mq_getattr (3)
and
.BR mq_setattr (3).
A process can request asynchronous notification
of the arrival of a message on a previously empty queue using
.BR mq_notify (3).

A message queue descriptor is a reference to an
.I "open message queue description"
(cf.
.BR open (2)).
After a
.BR fork (2),
a child inherits copies of its parent's message queue descriptors,
and these descriptors refer to the same open message queue descriptions
as the corresponding descriptors in the parent.
Corresponding descriptors in the two processes share the flags
.RI ( mq_flags )
that are associated with the open message queue description.

Each message has an associated
.IR priority ,
and messages are always delivered to the receiving process
highest priority first.
Message priorities range from 0 (low) to
.I sysconf(_SC_MQ_PRIO_MAX)\ -\ 1
(high).
On Linux,
.I sysconf(_SC_MQ_PRIO_MAX)
returns 32768, but POSIX.1-2001 requires only that
an implementation support at least priorities in the range 0 to 31;
some implementations provide only this range.
.PP
The remainder of this section describes some specific details
of the Linux implementation of POSIX message queues.
.SS Library interfaces and system calls
In most cases the
.BR mq_* ()
library interfaces listed above are implemented
on top of underlying system calls of the same name.
Deviations from this scheme are indicated in the following table:
.RS
.TS
lB lB
l l.
Library interface	System call
mq_close(3)	close(2)
mq_getattr(3)	mq_getsetattr(2)
mq_notify(3)	mq_notify(2)
mq_open(3)	mq_open(2)
mq_receive(3)	mq_timedreceive(2)
mq_send(3)	mq_timedsend(2)
mq_setattr(3)	mq_getsetattr(2)
mq_timedreceive(3)	mq_timedreceive(2)
mq_timedsend(3)	mq_timedsend(2)
mq_unlink(3)	mq_unlink(2)
.TE
.RE
.SS Versions
POSIX message queues have been supported on Linux since kernel 2.6.6.
Glibc support has been provided since version 2.3.4.
.SS Kernel configuration
Support for POSIX message queues is configurable via the
.B CONFIG_POSIX_MQUEUE
kernel configuration option.
This option is enabled by default.
.SS Persistence
POSIX message queues have kernel persistence:
if not removed by
.BR mq_unlink (3),
a message queue will exist until the system is shut down.
.SS Linking
Programs using the POSIX message queue API must be compiled with
.I cc \-lrt
to link against the real-time library,
.IR librt .
.SS /proc interfaces
The following interfaces can be used to limit the amount of
kernel memory consumed by POSIX message queues:
.TP
.I /proc/sys/fs/mqueue/msg_max
This file can be used to view and change the ceiling value for the
maximum number of messages in a queue.
This value acts as a ceiling on the
.I attr\->mq_maxmsg
argument given to
.BR mq_open (3).
The default value for
.I msg_max
is 10.
The minimum value is 1 (10 in kernels before 2.6.28).
The upper limit is
.BR HARD_MSGMAX .
The
.I msg_max
limit is ignored for privileged processes
.RB ( CAP_SYS_RESOURCE ),
but the
.BR HARD_MSGMAX
ceiling is nevertheless imposed.
The definition of
.BR HARD_MSGMAX
has changed across kernel versions:
before Linux 2.6.33, it was
.IR "(131072\ /\ sizeof(void\ *))"
(32768 on Linux/86);
from Linux 2.6.33, it was
.IR "(32768\ *\ sizeof(void\ *)/4)" ;
since Linux 3.5,
.\" commit 5b5c4d1a1440e94994c73dddbad7be0676cd8b9a
it is 65,536.
.TP
.I /proc/sys/fs/mqueue/msgsize_max
This file can be used to view and change the ceiling on the
maximum message size.
This value acts as a ceiling on the
.I attr\->mq_msgsize
argument given to
.BR mq_open (3).
The default value for
.I msgsize_max
is 8192 bytes.
The minimum value is 128 (8192 in kernels before 2.6.28).
The upper limit for
.I msgsize_max
has varied across kernel versions:
before Linux 2.6.28, the upper limit was
.BR INT_MAX ;
(that is, 2,147,483,647 on Linux/86);
starting with Linux 2.6.38, the limit was 1,048,576;
since Linux 3.5, the limit is 16,777,216
.RB ( HARD_MSGSIZEMAX ).
The
.I msgsize_max
limit is ignored for privileged process
.RB ( CAP_SYS_RESOURCE ),
but, since Linux 3.5, the 
.BR HARD_MSGSIZEMAX
ceiling is enforced for privileged processes.
.TP
.I /proc/sys/fs/mqueue/queues_max
This file can be used to view and change the system-wide limit on the
number of message queues that can be created.
Only privileged processes
.RB ( CAP_SYS_RESOURCE )
can create new message queues once this limit has been reached.
The default value for
.I queues_max
is 256; it can be changed to any value in the range 0 to INT_MAX.
.SS Resource limit
The
.B RLIMIT_MSGQUEUE
resource limit, which places a limit on the amount of space
that can be consumed by all of the message queues
belonging to a process's real user ID, is described in
.BR getrlimit (2).
.SS Mounting the message queue filesystem
On Linux, message queues are created in a virtual filesystem.
(Other implementations may also provide such a feature,
but the details are likely to differ.)
This filesystem can be mounted (by the superuser) using the following
commands:
.in +4n
.nf

.RB "#" " mkdir /dev/mqueue"
.RB "#" " mount \-t mqueue none /dev/mqueue"

.fi
.in
The sticky bit is automatically enabled on the mount directory.

After the filesystem has been mounted, the message queues on the system
can be viewed and manipulated using the commands usually used for files
(e.g.,
.BR ls (1)
and
.BR rm (1)).

The contents of each file in the directory consist of a single line
containing information about the queue:
.in +4n
.nf

.RB "$" " cat /dev/mqueue/mymq"
QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260

.fi
.in
These fields are as follows:
.TP
.B QSIZE
Number of bytes of data in all messages in the queue.
.TP
.B NOTIFY_PID
If this is nonzero, then the process with this PID has used
.BR mq_notify (3)
to register for asynchronous message notification,
and the remaining fields describe how notification occurs.
.TP
.B NOTIFY
Notification method:
0 is
.BR SIGEV_SIGNAL ;
1 is
.BR SIGEV_NONE ;
and
2 is
.BR SIGEV_THREAD .
.TP
.B SIGNO
Signal number to be used for
.BR SIGEV_SIGNAL .
.SS Polling message queue descriptors
On Linux, a message queue descriptor is actually a file descriptor,
and can be monitored using
.BR select (2),
.BR poll (2),
or
.BR epoll (7).
This is not portable.
.SS IPC namespaces
For a discussion of the interaction of System V IPC objects and
IPC namespaces, see
.BR namespaces (7).
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
System V message queues
.RB ( msgget (2),
.BR msgsnd (2),
.BR msgrcv (2),
etc.) are an older API for exchanging messages between processes.
POSIX message queues provide a better designed interface than
System V message queues;
on the other hand POSIX message queues are less widely available
(especially on older systems) than System V message queues.

Linux does not currently (2.6.26) support the use of access control
lists (ACLs) for POSIX message queues.
.SH EXAMPLE
An example of the use of various message queue functions is shown in
.BR mq_notify (3).
.SH SEE ALSO
.BR getrlimit (2),
.BR mq_getsetattr (2),
.BR poll (2),
.BR select (2),
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_notify (3),
.BR mq_open (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR epoll (7),
.BR namespaces (7)