MQ(3) BSD Library Functions Manual MQ(3)NAME
mq, mqueue — POSIX message queues (REALTIME)
LIBRARY
POSIX Real-time Library (librt, -lrt)
SYNOPSIS
#include <mqueue.h>
DESCRIPTION
The IEEE Std 1003.1-2001 (“POSIX.1”) standard defines and NetBSD imple‐
ments an interprocess communication (IPC) interface known as POSIX mes‐
sage queues. Although the basic functionality is similar, mq is distinct
from the older AT&T System V UNIX message queues (see for example ipcs(1)
or msgget(2)).
Rationale
The rationale behind mq is to provide an efficient, priority-driven asyn‐
chronous 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 perfor‐
mance 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:
1. Unlike the predecessors, mq provides an asynchronous notification
mechanism.
2. Messages are prioritized. The queue always remains sorted such
that the oldest message of the highest priority is always received
first, regardless of the number of messages in the queue.
3. By default, the functions to send and receive messages are block‐
ing calls. It is however possible to use non-blocking variants
with mq. Furthermore, it is possible to specify timeouts to avoid
non-deterministic blocking.
4. Resource limits can be enforced -- or perhaps more importantly,
the availability of resources can be ensured as the internal data
structures are preallocated.
Descriptors and Naming
Comparable to pipes and FIFOs (a.k.a. named pipes), all POSIX message
queue operations are performed by using a descriptor. The used type is
mqd_t, an abbreviation from a “message queue descriptor”. In the NetBSD
implementation this is actually an ordinary file descriptor. This means
that it is possible, but not portable, to monitor a message queue
descriptor by using poll(2) or select(2).
Message queues are named by character strings that represent (absolute)
pathnames. The used interface is analogous to the conventional file con‐
cepts. 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).
Processes
When a new process is created or the program is terminated, message
queues behave like files. More specifically, when fork(2) is called,
files and message queues are inherited, and when the program terminates
by calling exit(3) or _exit(2), both file descriptors and message queues
are closed. However, the exec(3) family of functions behave somewhat
differently for message queues and files: all message queues are closed
when a process calls one of the exec() functions. In this respect POSIX
message queues are closer to FIFOs than normal pipes.
Attributes
All message queues have an attribute associated with them. This is rep‐
resented by the mq_attr structure:
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 mes‐
sages (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.
Asynchronous Notification
Instead of blocking in the functions that receive messages, mq offers an
asynchronous mechanism for a process to receive notifications that mes‐
sages 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 inter‐
rupted by a signal. This may be a limitation for some realtime applica‐
tions.
Priorities
Each message has a priority, ranging from 0 to the implementation-defined
MQ_PRIO_MAX. The POSIX standard enforces the minimum value of the maxi‐
mum 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 fol‐
lows 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.
FUNCTIONS
The following functions are available in the API.
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
COMPATIBILITY
Despite of some early fears, the POSIX message queue implementations are
fairly compatible with each other. Nevertheless, few points can be noted
for portable applications.
· It is not portable to use functions external to the API with message
queue descriptors.
· The standard leaves the rules loose with respect to the message queue
names. Only the interpretation of the first slash character is con‐
sistent; the following slash characters may or may not follow the
conventional construction rules for a pathname.
· The length limits for a message queue name are implementation-
defined. These may or may not follow the conventional pathname lim‐
its PATH_MAX and NAME_MAX.
SEE ALSO
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.
STANDARDS
The POSIX message queue implementation is expected to conform to IEEE Std
1003.1-2001 (“POSIX.1”).
HISTORY
The POSIX message queue API first appeared in NetBSD 5.0.
CAVEATS
User should be careful to unlink message queues at the program termina‐
tion. Otherwise it is possible to leave them lying around.
BSD July 28, 2010 BSD
