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       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
       identified  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
       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
       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
       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 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
       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_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  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
       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  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

              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  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).

              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 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

           $ 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
       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:

           $ 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.

              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
       available (especially on older systems) than System V message queues.


       An  example  of  the use of various message queue functions is shown in


       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)


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