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     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-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 asso-
          ciated 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.

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        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 fol-
          lowing table:
               lB lB l l.  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 ker-
          nel 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.

        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:

               htmlmanrefend/proc/sys/fs/mqueue/msg_default(sinceLinux3

                   htmlmanrefstart/proc/sys/fs/mqueue/msg_max.htmlmanrefend/proc/sys/fs/mqueue/msg_max.

               htmlmanrefstartmsg_defaulthtmlmanrefendmsg_default htmlmanrefstartmsg_max.htmlmanrefendmsg_max.

                      htmlmanrefstart131072/sizeof(void*)htmlmanrefend131072/sizeof(void*)

                      htmlmanrefstart(32768*sizeof(void*)/4) htmlmanrefend(32768*sizeof(void*)/4)

          htmlmanrefstart/proc/sys/fs/mqueue/msgsize_default(sinceLinux3.5)    .}f 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 ker-
               nel 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 pro-
               cesses.

          /proc/sys/fs/mqueue/queues_max

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               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 mes-
          sage 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 filesys-
          tem.  (Other implementations may also provide such a fea-
          ture, but the details are likely to differ.)  This filesys-
          tem 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 direc-
          tory.

          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 sin-
          gle 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 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.

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          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 implementa-
          tion.)  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 ipc_namespaces(7).

     NOTES
          System V message queues (msgget(2), msgsnd(2), msgrcv(2),
          etc.) are an older API for exchanging messages between pro-
          cesses.  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.

     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 rec-
          tified 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.

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

     SEE ALSO
          getrlimit(2), mq_getsetattr(2), poll(2), select(2),

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          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 5.10 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 https://www.kernel.org/doc/man-pages/.

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