MQ(3) | Library Functions Manual | MQ(3) |
mq
, mqueue
—
#include <mqueue.h>
mq
is distinct from
the older AT&T System V UNIX message queues
(see for example ipcs(1) or
msgget(2)).
mq
is to provide an efficient,
priority-driven asynchronous IPC mechanism. When the AT&T
System V UNIX message queues were first implemented, the
reasoning was similar: the only form of IPC was half-duplex pipes and message
queues were seen to overcome the performance limitations with these.
But arguably in modern systems there is little difference between
the efficiency of the System V message queues, pipes, and UNIX domain
sockets (if anything, the AT&T System V
UNIX message queues tend to be slower than the rest). The fundamental
performance bottleneck is however still there with
mq
as well: data must be first copied from the
sender to the kernel and then from the kernel to the receiver. The bigger
the message, the higher the overhead.
For realtime applications, mq
offers some
advantages:
mq
provides an
asynchronous notification mechanism.mq
. Furthermore, it is possible to specify
timeouts to avoid non-deterministic blocking.Message queues are named by character strings that represent
(absolute) pathnames. The used interface is analogous to the conventional
file concepts. But unlike FIFOs and pipes, neither POSIX nor System V
message queues are accessed by using
open(2),
read(2), or
write(2). Instead, equivalents
such as mq_open
(),
mq_close
(), and mq_unlink
()
are used.
The standard does not specify whether POSIX message queues are
exposed at the file system level. It can be argued that
mq
inherited an old problem with the System V
message queues. Even if an implementation would have support for it, it is
not portable to view message queues by
ls(1), remove these with
rm(1), or adjust the permissions
with chmod(1).
exec
()
functions. In this respect POSIX message queues are closer to FIFOs than
normal pipes.
struct mq_attr { long mq_flags; long mq_maxmsg; long mq_msgsize; long mq_curmsgs; };
The members in the structure are: flags set for the message queue (mq_flags), the maximum number of messages in the queue (mq_maxmsg), the maximum size of each message (mq_msgsize), and the number of queued messages (mq_curmsgs).
The overall resource requirements for a particular message queue
are given by mq_maxmsg and
mq_msgsize. These two can be specified when the queue
is created by a call to mq_open
(). The constraints
are enforced through the lifetime of the queue: an error is returned if a
message larger than mq_msgsize is sent, and if the
message queue is already full, as determined by
mq_maxmsg, the call to queue a message will either
block or error out.
Although there are two functions,
mq_getattr
() and
mq_setattr
(), to retrieve and set attributes,
resource limits cannot be changed once the queue has been created. In
NetBSD the super user may however control the global
resource limits by using few
sysctl(7) variables.
mq
offers an asynchronous mechanism for a process to
receive notifications that messages are available in the message queue. The
function mq_notify
() is used to register for
notification. Either a signal or a thread can be used as the type of
notification; see sigevent(3)
for details.
Bear in mind that no notification is sent for an arrival of a
message to a non-empty message queue. In other words,
mq_notify
() does not by itself ensure that a process
will be notified every time a message arrives. Thus, after having called
mq_notify
(), an application may need to repeatedly
call mq_receive
() until the queue is empty. This
requires that the message queue was created with the
O_NONBLOCK
flag; otherwise
mq_receive
() blocks until a message is again queued
or the call is interrupted by a signal. This may be a limitation for some
realtime applications.
MQ_PRIO_MAX
. The POSIX standard enforces the minimum
value of the maximum priority to be 32. All messages are inserted into a
message queue according to the specified priority. High priority messages are
sent before low priority messages. If the used priority is constant,
mq
follows the FIFO (First In, First Out) principle.
The basic rule of thumb with realtime prioritization is that low priority tasks should never unnecessarily delay high priority tasks. Priority inheritance is not however part of the provided API; the receiver process may run at low priority even when receiving high priority messages. To address this limitation and other potential realtime problems, the user may consider other functions from the POSIX Real-time Library (librt, -lrt). The process scheduling interface described in sched(3) can be mentioned as an example.
Function | Description |
mq_open(3) | open a message queue |
mq_close(3) | close a message queue |
mq_unlink(3) | remove a message queue |
mq_send(3) | send a message |
mq_receive(3) | receive a message |
mq_timedsend(3) | send a message with a timeout |
mq_timedreceive(3) | receive a message with a timeout |
mq_getattr(3) | get message queue attributes |
mq_setattr(3) | set message queue attributes |
mq_notify(3) | register asynchronous notify |
PATH_MAX
and
NAME_MAX
.Bill O. Gallmeister, POSIX.4: Programming for the Real World, O'Reilly and Associates, 1995.
Richard W. Stevens, UNIX Network Programming, Volume 2: Interprocess Communications, Prentice Hall, Second Edition, 1998.
July 28, 2010 | NetBSD 9.2 |