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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 (see
       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 message queue descriptors in the parent.   Corresponding  message  queue
       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 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 and to set the default attributes for new message queues:

       /proc/sys/fs/mqueue/msg_default (since Linux 3.5)
              This file defines the value used for a new queue's mq_maxmsg setting when the queue
              is  created with a call to mq_open(3) where attr is specified as NULL.  The default
              value  for  this  file   is   10.    The   minimum   and   maximum   are   as   for
              /proc/sys/fs/mqueue/msg_max.   A  new  queue's  default mq_maxmsg value will be the
              smaller of msg_default and msg_max.  Up until Linux 2.6.28, the  default  mq_maxmsg
              was  10;  from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
              msg_max limit.

              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_MSGMAX.  The
              msg_max limit is ignored  for  privileged  processes  (CAP_SYS_RESOURCE),  but  the
              HARD_MSGMAX ceiling is nevertheless imposed.

              The definition of HARD_MSGMAX has changed across kernel versions:

              *  Up to Linux 2.6.32: 131072 / sizeof(void *)

              *  Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)

              *  Since Linux 3.5: 65,536

       /proc/sys/fs/mqueue/msgsize_default (since Linux 3.5)
              This  file  defines  the  value  used for a new queue's mq_msgsize setting when the
              queue is created with a call to mq_open(3) where attr is specified  as  NULL.   The
              default  value  for  this file is 8192 (bytes).  The minimum and maximum are as for
              /proc/sys/fs/mqueue/msgsize_max.  If msgsize_default  exceeds  msgsize_max,  a  new
              queue's  default  mq_msgsize  value  is  capped to the msgsize_max limit.  Up until
              Linux 2.6.28, the default mq_msgsize was 8192; from Linux 2.6.28 to Linux 3.4,  the
              default was the value defined for the msgsize_max limit.

              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 has varied  across  kernel

              *  Before Linux 2.6.28, the upper limit is INT_MAX.

              *  From Linux 2.6.28 to 3.4, the limit is 1,048,576.

              *  Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEMAX).

              The  msgsize_max  limit  is ignored for privileged process (CAP_SYS_RESOURCE), but,
              since Linux 3.5, the HARD_MSGSIZEMAX ceiling is enforced for privileged processes.

              This file can be used to view and change the system-wide limit  on  the  number  of
              message  queues  that can be created.  The default value for queues_max is 256.  No
              ceiling is imposed on the queues_max limit; privileged processes (CAP_SYS_RESOURCE)
              can exceed the limit (but see BUGS).

   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 (but see BUGS).

              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.

   Linux implementation of message queue descriptors
       On  Linux,  a  message  queue  descriptor  is actually a file descriptor.  (POSIX does not
       require such an implementation.)  This means  that  a  message  queue  descriptor  can  be
       monitored using select(2), poll(2), or epoll(7).  This is not portable.

       The  close-on-exec flag (see open(2)) is automatically set on the file descriptor returned
       by mq_open(2).

   IPC namespaces
       For a discussion of the interaction of POSIX message queue objects and IPC namespaces, see


       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.


       In  Linux  versions  3.5 to 3.14, the kernel imposed a ceiling of 1024 (HARD_QUEUESMAX) on
       the value to which the queues_max limit could be raised, and the ceiling was enforced even
       for  privileged  processes.   This ceiling value was removed in Linux 3.14, and patches to
       stable kernels 3.5.x to 3.13.x also removed the ceiling.

       As originally implemented (and documented), the QSIZE field displayed the total number  of
       (user-supplied)  bytes  in  all  messages in the message queue.  Some changes in Linux 3.5
       inadvertently changed the behavior, so that this field also included  a  count  of  kernel
       overhead  bytes  used  to store the messages in the queue.  This behavioral regression was
       rectified in Linux 4.2 (and earlier stable kernel series), so that  the  count  once  more
       included just the bytes of user data in messages in the queue.


       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), namespaces(7)


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