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 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 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 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 prior-
       ities 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 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 CONFIG_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 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_msg-
	      size  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.

   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.

       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.

   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 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.

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 3.55 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)