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 de-
       scription (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_un-
       link(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_RE-
              SOURCE), but the HARD_MSGMAX ceiling is nevertheless imposed.

              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 ms-
              gsize_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 de-
              fault 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 POSIX message queue objects  and
       IPC namespaces, see ipc_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 5.05 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                             2019-10-10                    MQ_OVERVIEW(7)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2024 Hurricane Electric. All Rights Reserved.