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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 only requires an
implementation to support priorities in the range 0 to 31; some
implementations only provide 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 3.32 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)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface