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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
       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)

   Versions
       POSIX message queues have been supported since Linux 2.6.6.  glibc support has been provided since  glibc
       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:

       /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.  Before 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 argument  given  to  mq_open(3).   The
              default  value  for  msg_max  is  10.  The minimum value is 1 (10 before Linux 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 Linux 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.   Before  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 before Linux 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 Linux 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 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
       ipc_namespaces(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 (Linux 2.6.26) support the use of access control lists (ACLs) for POSIX  message
       queues.

BUGS

       Since  Linux  3.5  to  Linux  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 Linux 3.5.x to Linux 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.

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), 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)