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



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  functional-
       ity.

       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 con-
       sisting 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 (cf.  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 descriptors in the parent.  Corresponding 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.1-2001 only requires an implementation to support  priorities
       in the range 0 to 31; some implementations only provide 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 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 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:

       /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
              and  minimum  value  for msg_max is 10; the upper limit is HARD_MAX: (131072 / sizeof(void *)) (32768 on
              Linux/86).  This limit is ignored for privileged processes (CAP_SYS_RESOURCE), but the HARD_MAX  ceiling
              is nevertheless imposed.

       /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 and minimum  value  for  msg-
              size_max  is 8192 bytes; the upper limit is INT_MAX (2147483647 on Linux/86).  This limit is ignored for
              privileged processes (CAP_SYS_RESOURCE).

       /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.  Only privileged processes (CAP_SYS_RESOURCE) can create new message queues once this limit
              has been reached.  The default value for queues_max is 256; it can be changed to any value in the  range
              0 to INT_MAX.

   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 file system
       On Linux, message queues are created in a virtual file system.  (Other implementations may also provide such  a
       feature,  but  the details are likely to differ.)  This file system 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 file system 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.

       NOTIFY_PID
              If  this  is non-zero, 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.

   Polling message queue descriptors
       On Linux, a message queue descriptor is actually a file descriptor,  and  can  be  monitored  using  select(2),
       poll(2), or epoll(7).  This is not portable.

CONFORMING TO
       POSIX.1-2001.

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 (2.6.26) support the use of access control lists (ACLs) for POSIX message queues.

EXAMPLE
       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)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-07-25                    MQ_OVERVIEW(7)