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.TH MQ_OVERVIEW 7 2006-02-25 "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 .
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
.IR "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 only requires
an implementation to support priorities in the range 0 to 31;
some implementations only provide 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
.B 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:
.in +0.25i
.TS
lB lB
l l.
Library interface	System call
mq_close(3)	close(2)
mq_getattr(3)	mq_getsetattr(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
.in -0.25i
.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 and minimum value for
.I msg_max
is 10; the upper limit is HARD_MAX:
.IR "(131072\ /\ sizeof(void\ *))"
(32768 on Linux/86).
This limit is ignored for privileged processes
.RB ( CAP_SYS_RESOURCE ),
but the HARD_MAX ceiling is nevertheless imposed.
.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 and minimum value for
.I msgsize_max
is 8192 bytes; the upper limit is INT_MAX
(2147483647 on Linux/86).
This limit is ignored for privileged processes
.RB ( CAP_SYS_RESOURCE ).
.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
.BR 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 file system
On Linux, message queues are created in a virtual file system.
(Other implementations may also provide such a feature,
but the details are likely to differ.)
This file system can be mounted using the following commands:
.in +0.25i
.nf

$ mkdir /dev/mqueue
$ mount -t mqueue none /dev/mqueue

.fi
.in -0.25i
The sticky bit is automatically enabled on the mount directory.

After the file system 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 +0.25i
.nf

$ ls /dev/mqueue/mymq
QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260
$ mount -t mqueue none /dev/mqueue

.fi
.in -0.25i
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 non-zero, 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.
.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.
.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 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 poll (2),
.BR select (2),
.BR epoll (4)