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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 identified by a name of the form /somename; that is, a null-termi‐
       nated string of up to NAME_MAX (i.e., 255) characters consisting of an initial slash, fol‐
       lowed  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 descrip‐
       tors,  and these descriptors refer to the same open message queue descriptions as the cor‐
       responding 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 receiv‐
       ing  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  under‐
       lying  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)

   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 CONFIG_POSIX_MQUEUE kernel con‐
       figuration 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  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.

       /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  argu‐
              ment  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.

       /proc/sys/fs/mqueue/msgsize_max
              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
              versions:

              *  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.

       /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.  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).

       NOTIFY_PID
              If this is nonzero, then the process with this PID has used mq_notify(3) to  regis‐
              ter  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.

   IPC namespaces
       For a discussion of the interaction of System V IPC objects and IPC namespaces, see names‐
       paces(7).

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 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.

EXAMPLE
       An example of the use of various message queue functions is shown in mq_notify(3).

BUGS
       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.

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), namespaces(7)

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2015-08-08                             MQ_OVERVIEW(7)

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