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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(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  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:

           lB lB l l.  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_time-
           dreceive(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   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(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 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.

       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 3.05 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)