mq_overview(7) — Linux manual page


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

NAME         top

       mq_overview - overview of POSIX message queues

DESCRIPTION         top

       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 /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 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

       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 descriptors in the two processes share the flags
       (mq_flags) that are associated with the open message queue

       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
       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
       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)

       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.

       POSIX message queues have kernel persistence: if not removed by
       mq_unlink(3), a message queue will exist until the system is shut

       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 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 limit.

              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 imposed.

              The definition of HARD_MSGMAX has changed across kernel

              *  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
              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 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 sys‐
       tem 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 notifica‐
              tion, and the remaining fields describe how notification

       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).

NOTES         top

       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

       Linux does not currently (2.6.26) support the use of access control
       lists (ACLs) for POSIX message queues.

BUGS         top

       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
       message queue.  Some changes in Linux 3.5 inadvertently changed the
       behavior, 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.

EXAMPLES         top

       An example of the use of various message queue functions is shown in

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of release 5.08 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                            2020-06-09                   MQ_OVERVIEW(7)

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