MQ_OVERVIEW(7)



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().

       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 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.

   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)

LINUX SPECIFIC DETAILS
   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(),  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.

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

SEE ALSO
       getrlimit(2),     mq_getsetattr(2),     mq_close(3),     mq_getattr(3),
       mq_notify(3),  mq_open(3),  mq_receive(3),  mq_send(3),   mq_unlink(3),
       poll(2), select(2), epoll(4)

Linux 2.6.16                      2006-02-25                    MQ_OVERVIEW(7)

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