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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

       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; 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

       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
       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 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)

       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 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.

	      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
	      value  for  msgsize_max is 8192 bytes.  The minimum value is 128
	      (8192 in kernels before  2.6.28).	  The  upper  limit  for  msg‐
	      size_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 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 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 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 3.65 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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