| NAME | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON | The Linux Programming Interface |
MQ_OVERVIEW(7) Linux Programmer's Manual MQ_OVERVIEW(7)
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:
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)
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:
/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_MAX:
(131072 / sizeof(void *)) (32768 on Linux/86). This limit is
ignored for privileged processes (CAP_SYS_RESOURCE), but the
HARD_MAX ceiling is nevertheless imposed.
/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 is 1,048,576 (in kernels before 2.6.28, the upper
limit was INT_MAX; that is, 2,147,483,647 on Linux/86). This
limit is ignored for privileged processes (CAP_SYS_RESOURCE).
/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 (CAP_SYS_RESOURCE) can create new message
queues once this limit has been reached. The default value
for queues_max is 256; it can be changed to any value in the
range 0 to INT_MAX.
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 file system.
(Other implementations may also provide such a feature, but the
details are likely to differ.) This file system 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 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., 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/mymq
QSIZE: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.
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).
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)
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2009-09-27 MQ_OVERVIEW(7)
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