Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


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
     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 4.16 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)

/data/webs/external/dokuwiki/data/pages/man/mq_overview.txt · Last modified: 2019/05/17 09:32 by

Was this page helpful?-10+1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki