MQ_OVERVIEW(7) manual page
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mq_overview - overview of POSIX message queues
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 (msgget(2)
, msgsnd(2)
, msgrcv(2)
, etc.), but provides similar functionality.
Message queues are created and opened using mq_open(3)
; this function
returns a message queue descriptor (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 /somename; that is, a null-terminated string of up to 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 mq_open(3)
.
Messages are transferred
to and from a queue using mq_send(3)
and mq_receive(3)
. When a process has
finished using the queue, it closes it using mq_close(3)
, and when the
queue is no longer required, it can be deleted using mq_unlink(3)
. Queue
attributes can be retrieved and (in some cases) modified using mq_getattr(3)
and mq_setattr(3)
. A process can request asynchronous notification of the
arrival of a message on a previously empty queue using mq_notify(3)
.
A
message queue descriptor is a reference to an open message queue description
(cf. open(2)
). After a 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 (mq_flags) that are associated
with the open message queue description.
Each message has an associated
priority, and messages are always delivered to the receiving process highest
priority first. Message priorities range from 0 (low) to sysconf(_SC_MQ_PRIO_MAX) - 1
(high). On Linux, 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.
The remainder
of this section describes some specific details of the Linux implementation
of POSIX message queues.
In most cases
the 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:
POSIX
message queues have been supported on Linux since kernel 2.6.6. Glibc support
has been provided since version 2.3.4.
Support for POSIX
message queues is configurable via the CONFIG_POSIX_MQUEUE kernel configuration
option. This option is enabled by default.
POSIX message queues
have kernel persistence: if not removed by mq_unlink(3)
, a message queue
will exist until the system is shut down.
Programs using the POSIX
message queue API must be compiled with cc -lrt to link against the real-time
library, librt.
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:
- /proc/sys/fs/mqueue/msg_default
(since Linux 3.5)
- This file defines the value used for a new queue’s mq_maxmsg
setting when the queue is created with a call to mq_open(3)
where attr
is specified as NULL. The default value for this file is 10. The minimum
and maximum are as for /proc/sys/fs/mqueue/msg_max. A new queue’s default
mq_maxmsg value will be the smaller of msg_default and msg_max. Up until
Linux 2.6.28, the default mq_maxmsg was 10; from Linux 2.6.28 to Linux 3.4,
the default was the value defined for the msg_max limit.
- /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 attr->mq_maxmsg
argument given to mq_open(3)
. The default value for msg_max is 10. The minimum
value is 1 (10 in kernels before 2.6.28). The upper limit is HARD_MSGMAX. The
msg_max limit is ignored for privileged processes (CAP_SYS_RESOURCE), but
the HARD_MSGMAX ceiling is nevertheless imposed.
The definition of HARD_MSGMAX
has changed across kernel versions:
- *
- Up to Linux 2.6.32: 131072 / sizeof(void *)
- *
- Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)
- *
- Since Linux 3.5:
65,536
- /proc/sys/fs/mqueue/msgsize_default
(since Linux 3.5)
- This file defines the value used for a new queue’s mq_msgsize
setting when the queue is created with a call to mq_open(3)
where attr
is specified as NULL. The default value for this file is 8192 (bytes). The
minimum and maximum are as for /proc/sys/fs/mqueue/msgsize_max. If msgsize_default
exceeds msgsize_max, a new queue’s default mq_msgsize value is capped to
the msgsize_max limit. Up until Linux 2.6.28, the default mq_msgsize was 8192;
from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
msgsize_max limit.
- /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 attr->mq_msgsize argument given to mq_open(3)
. The default
value for msgsize_max is 8192 bytes. The minimum value is 128 (8192 in kernels
before 2.6.28). The upper limit for msgsize_max has varied across kernel versions:
- *
- Before Linux 2.6.28, the upper limit is INT_MAX.
- *
- From Linux 2.6.28 to 3.4,
the limit is 1,048,576.
- *
- Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEMAX).
- The
- msgsize_max limit is ignored for privileged process (CAP_SYS_RESOURCE),
but, since Linux 3.5, the HARD_MSGSIZEMAX ceiling is enforced for privileged
processes.
- /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 queues_max is 256. No ceiling is imposed
on the queues_max limit; privileged processes (CAP_SYS_RESOURCE) can exceed
the limit (but see BUGS).
The 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 getrlimit(2)
.
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:
# mkdir /dev/mqueue# mount -t mqueue none /dev/mqueue
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., ls(1)
and rm(1)
).
The contents
of each file in the directory consist of a single line containing information
about the queue:
$ cat /dev/mqueue/mymqQSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
These fields are as follows:
- QSIZE
- Number of bytes of data in all messages
in the queue.
- NOTIFY_PID
- If this is nonzero, then the process with this
PID has used mq_notify(3)
to register for asynchronous message notification,
and the remaining fields describe how notification occurs.
- NOTIFY
- Notification
method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2 is SIGEV_THREAD.
- SIGNO
- Signal number to be used for SIGEV_SIGNAL.
On
Linux, a message queue descriptor is actually a file descriptor, and can
be monitored using select(2)
, poll(2)
, or epoll(7)
. This is not portable.
For a discussion of the interaction of System V IPC objects
and IPC namespaces, see namespaces(7)
.
POSIX.1-2001.
System
V message queues (msgget(2)
, msgsnd(2)
, 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.
An example of the use of various message queue functions is shown
in mq_notify(3)
.
In Linux versions 3.5 to 3.14, the kernel imposed a ceiling
of 1024 (HARD_QUEUESMAX) on the value to which the 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.
getrlimit(2)
, mq_getsetattr(2)
,
poll(2)
, select(2)
, mq_close(3)
, mq_getattr(3)
, mq_notify(3)
, mq_open(3)
,
mq_receive(3)
, mq_send(3)
, mq_unlink(3)
, epoll(7)
, namespaces(7)
This
page is part of release 3.78 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 http://www.kernel.org/doc/man-pages/.
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