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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 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 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

       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  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)

       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

              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 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.

              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.




       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 mq_notify(3).


       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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