man7/mq_overview.7

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.TH MQ_OVERVIEW 7 2020-06-09 "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.
.PP
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).
.PP
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).
.PP
A message queue descriptor is a reference to an
.I "open message queue description"
(see
.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 message queue descriptors in the parent.
Corresponding message queue descriptors in the two processes share the flags
.RI ( mq_flags )
that are associated with the open message queue description.
.PP
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 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 and to set
the default attributes for new message queues:
.TP
.IR /proc/sys/fs/mqueue/msg_default " (since Linux 3.5)"
This file defines the value used for a new queue's
.I mq_maxmsg
setting when the queue is created with a call to
.BR mq_open (3)
where
.I attr
is specified as NULL.
The default value for this file is 10.
The minimum and maximum are as for
.IR /proc/sys/fs/mqueue/msg_max .
A new queue's default
.I mq_maxmsg
value will be the smaller of
.IR msg_default
and
.IR msg_max .
Up until Linux 2.6.28, the default
.I mq_maxmsg
was 10;
from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
.I msg_max
limit.
.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.
.IP
The definition of
.BR HARD_MSGMAX
has changed across kernel versions:
.RS
.IP * 3
Up to Linux 2.6.32:
.IR "131072\ /\ sizeof(void\ *)"
.IP *
Linux 2.6.33 to 3.4:
.IR "(32768\ *\ sizeof(void\ *) / 4)"
.IP *
Since Linux 3.5:
.\" commit 5b5c4d1a1440e94994c73dddbad7be0676cd8b9a
65,536
.RE
.TP
.IR /proc/sys/fs/mqueue/msgsize_default " (since Linux 3.5)"
This file defines the value used for a new queue's
.I mq_msgsize
setting when the queue is created with a call to
.BR mq_open (3)
where
.I attr
is specified as NULL.
The default value for this file is 8192 (bytes).
The minimum and maximum are as for
.IR /proc/sys/fs/mqueue/msgsize_max .
If
.IR msgsize_default
exceeds
.IR msgsize_max ,
a new queue's default
.I mq_msgsize
value is capped to the
.I msgsize_max
limit.
Up until Linux 2.6.28, the default
.I mq_msgsize
was 8192;
from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
.I msgsize_max
limit.
.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:
.RS
.IP * 3
Before Linux 2.6.28, the upper limit is
.BR INT_MAX .
.IP *
From Linux 2.6.28 to 3.4, the limit is 1,048,576.
.IP *
Since Linux 3.5, the limit is 16,777,216
.RB ( HARD_MSGSIZEMAX ).
.RE
.IP
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.
The default value for
.I queues_max
is 256.
No ceiling is imposed on the
.I queues_max
limit; privileged processes
.RB ( CAP_SYS_RESOURCE )
can exceed the limit (but see BUGS).
.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:
.PP
.in +4n
.EX
.RB "#" " mkdir /dev/mqueue"
.RB "#" " mount \-t mqueue none /dev/mqueue"
.EE
.in
.PP
The sticky bit is automatically enabled on the mount directory.
.PP
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)).
.PP
The contents of each file in the directory consist of a single line
containing information about the queue:
.PP
.in +4n
.EX
.RB "$" " cat /dev/mqueue/mymq"
QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260
.EE
.in
.PP
These fields are as follows:
.TP
.B QSIZE
Number of bytes of data in all messages in the queue (but see BUGS).
.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 Linux implementation of message queue descriptors
On Linux, a message queue descriptor is actually a file descriptor.
(POSIX does not require such an implementation.)
This means that a message queue descriptor can be monitored using
.BR select (2),
.BR poll (2),
or
.BR epoll (7).
This is not portable.
.PP
The close-on-exec flag (see
.BR open (2))
is automatically set on the file descriptor returned by
.BR mq_open (2).
.SS IPC namespaces
For a discussion of the interaction of POSIX message queue objects and
IPC namespaces, see
.BR ipc_namespaces (7).
.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.
.PP
Linux does not currently (2.6.26) support the use of access control
lists (ACLs) for POSIX message queues.
.SH BUGS
In Linux versions 3.5 to 3.14, the kernel imposed a ceiling of 1024
.RB ( HARD_QUEUESMAX )
on the value to which the
.I queues_max
limit could be raised,
and the ceiling was enforced even for privileged processes.
This ceiling value was removed in Linux 3.14,
and patches to stable kernels 3.5.x to 3.13.x also removed the ceiling.
.PP
As originally implemented (and documented),
the QSIZE field displayed the total number of (user-supplied)
bytes in all messages in the message queue.
Some changes in Linux 3.5
.\" commit d6629859b36d
inadvertently changed the behavior,
so that this field also included a count of kernel overhead bytes
used to store the messages in the queue.
This behavioral regression was rectified in Linux 4.2
.\" commit de54b9ac253787c366bbfb28d901a31954eb3511
(and earlier stable kernel series),
so that the count once more included just the bytes of user data
in messages in the queue.
.SH EXAMPLES
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)