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Linux 2.6 - man page for mq_overview (linux section 7)

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-termi-
       nated string of up to NAME_MAX (i.e., 255) characters consisting of an initial slash, fol-
       lowed  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 descrip-
       tors,  and these descriptors refer to the same open message queue descriptions as the cor-
       responding 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 receiv-
       ing  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.

   Library interfaces and system calls
       In most cases the mq_*() library interfaces listed above are implemented on top of  under-
       lying  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.

   Kernel configuration
       Support	for  POSIX message queues is configurable via the CONFIG_POSIX_MQUEUE kernel con-
       figuration 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  memory  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  argu-
	      ment  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 privi-
	      leged processes  (CAP_SYS_RESOURCE),  but  the  HARD_MAX	ceiling  is  nevertheless

	      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).

	      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.

   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 filesystem
       On  Linux, message queues are created in a virtual filesystem.  (Other implementations may
       also provide such a feature, but the details are likely to differ.)  This  filesystem  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 filesystem 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.

	      If  this is nonzero, then the process with this PID has used mq_notify(3) to regis-
	      ter 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.


       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 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2009-09-27				   MQ_OVERVIEW(7)

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