mq_overview(7) — Linux manual page


mq_overview(7)      Miscellaneous Information 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 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 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 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 since Linux 2.6.6.
       glibc support has been provided since glibc 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.  Before 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 before Linux 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 Linux 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.  Before 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 before Linux 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 Linux 3.4, the limit is 1,048,576.

              •  Since Linux 3.5, the limit is 16,777,216

              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

              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

   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 BUGS).

              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).

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 queues.

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

BUGS         top

       Since Linux 3.5 to Linux 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 Linux 3.5.x to Linux 3.13.x also removed the

       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 mq_notify(3).

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)

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Linux man-pages 6.9.1          2024-05-02                 mq_overview(7)

Pages that refer to this page: execve(2)fork(2)intro(2)mq_getsetattr(2)msgctl(2)msgget(2)msgop(2)mq_close(3)mq_getattr(3)mq_notify(3)mq_open(3)mq_receive(3)mq_send(3)mq_unlink(3)sd_is_fifo(3)proc_sys_fs(5)systemd.exec(5)systemd.socket(5)capabilities(7)ipc_namespaces(7)namespaces(7)sysvipc(7)