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NAME | DESCRIPTION | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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mq_overview(7) Miscellaneous Information 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 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)
Versions
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.
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 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.
/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 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
versions:
• 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.
/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 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
(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.
/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.
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 BUGS).
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.
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 (Linux 2.6.26) support the use of access
control lists (ACLs) for POSIX message queues.
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
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.
An example of the use of various message queue functions is shown
in mq_notify(3).
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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