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MQ_OVERVIEW(7)		   Linux Programmer's Manual		MQ_OVERVIEW(7)

NAME
       mq_overview - Overview of POSIX message queues

DESCRIPTION
       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  identi‐
       fied by a name of the form /somename.  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 mes‐
       sage 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  pro‐
       cesses  share  the  flags  (mq_flags) that are associated with the open
       message queue description.

       Each message has an associated priority, and messages are always deliv‐
       ered  to the receiving process highest priority first.  Message priori‐
       ties 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.

   Library interfaces and system calls
       In most cases the mq_*() library interfaces  listed  above  are	imple‐
       mented  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)

   Versions
       POSIX message queues have been supported on Linux since	kernel	2.6.6.
       Glibc support has been provided since version 2.3.4.

   Kernel configuration
       Support	 for  POSIX  message  queues  is  configurable	via  the  CON‐
       FIG_POSIX_MQUEUE kernel configuration option.  This option  is  enabled
       by default.

   Persistence
       POSIX  message  queues  have  kernel  persistence:  if  not  removed by
       mq_unlink(3), a message queue will exist until the system is shut down.

   Linking
       Programs using the POSIX message queue API must	be  compiled  with  cc
       -lrt to link against the real-time library, librt.

   /proc interfaces
       The following interfaces can be used to limit the amount of kernel mem‐
       ory 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 and minimum value for msg_max is 10; 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 max‐
	      imum message  size.   This  value	 acts  as  a  ceiling  on  the
	      attr->mq_msgsize	argument given to mq_open(3).  The default and
	      minimum value for msgsize_max is 8192 bytes; the upper limit  is
	      INT_MAX  (2147483647  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 priv‐
	      ileged  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.

   Resource limit
       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).

   Mounting the message queue file system
       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 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  sys‐
       tem  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:

	   $ ls /dev/mqueue/mymq
	   QSIZE:129	 NOTIFY:2    SIGNO:0	NOTIFY_PID:8260
	   $ mount -t mqueue none /dev/mqueue

       These fields are as follows:

       QSIZE  Number of bytes of data in all messages in the queue.

       NOTIFY_PID
	      If  this	is  non-zero,  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.

   Polling message queue descriptors
       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.

CONFORMING TO
       POSIX.1-2001.

NOTES
       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 avail‐
       able (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.

EXAMPLE
       An  example  of	the use of various message queue functions is shown in
       mq_notify(3).

SEE ALSO
       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)

COLOPHON
       This page is part of release 3.15 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				  2008-09-29			MQ_OVERVIEW(7)
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