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 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 /some-
       name; 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 (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  imple-
       mentations 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 value for msg_max is 10.
              The  minimum  value  is  1  (10 in kernels before 2.6.28).  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 value for msgsize_max is 8192 bytes.  The  minimum
              value  is  128  (8192  in kernels before 2.6.28).  The upper limit for msgsize_max is 1,048,576 (in kernels before
              2.6.28, the upper limit was INT_MAX; that is, 2,147,483,647 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 cre-
              ated.  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 mes-
       sage 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 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.

   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  pro-
       cesses.   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 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-09-27                                             MQ_OVERVIEW(7)