MQ_OVERVIEW(7)             Linux Programmer's Manual            MQ_OVERVIEW(7)

       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

       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  identi-
       fied 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

       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  mes-
       sage 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 descrip-
       tors in the two processes share the flags (mq_flags) that  are  associ-
       ated with the open message queue description.

       Each message has an associated priority, and messages are always deliv-
       ered to the receiving process highest priority first.  Message  priori-
       ties  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 imple-
       mented 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   CON-
       FIG_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 mem-
       ory  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

              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

              The  definition  of  HARD_MSGMAX  has changed across kernel ver-

              *  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 max-
              imum  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  msg-
              size_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.

              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

              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 System V IPC objects and IPC
       namespaces, see namespaces(7).

       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 avail-
       able (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  mes-
       sage queue.  Some changes in Linux 3.5 inadvertently changed the behav-
       ior, 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

       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)

       This  page  is  part of release 4.15 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

Linux                             2017-09-15                    MQ_OVERVIEW(7)
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