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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; 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
       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_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
       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 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
              maximum 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
              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).

              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 (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 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:

           $ 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
       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.


       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)


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