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 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 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 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 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

              The definition of HARD_MSGMAX has changed across kernel versions:

              *  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 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

              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

       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).

       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.

       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

       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 5.13 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)