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MQ_OVERVIEW(7)		   Linux Programmers 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

       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 pro
       cess  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 parents 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_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.

   Kernel configuration
       Support	 for  POSIX  message  queues  is  configurable	via  the  CON
       FIG_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.

   /proc interfaces
       The following interfaces can be used to limit the amount of kernel mem
       ory consumed by POSIX message queues:

	      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

	      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).

	      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 processs 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

	   $ 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.

	      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


       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.

       An example of the use of various message queue functions  is  shown  in

       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 3.05 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				  2008-06-15			MQ_OVERVIEW(7)

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