Section (7) mq_overview
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 /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_zsingle_quotesz_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 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 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_zsingle_quotesz_s
mq_maxmsg
setting when the queue is created with a call to mq_open(3) whereattr
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_zsingle_quotesz_s defaultmq_maxmsg
value will be the smaller ofmsg_default
andmsg_max
. Up until Linux 2.6.28, the defaultmq_maxmsg
was 10; from Linux 2.6.28 to Linux 3.4, the default was the value defined for themsg_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 formsg_max
is 10. The minimum value is 1 (10 in kernels before 2.6.28). The upper limit isHARD_MSGMAX
. Themsg_max
limit is ignored for privileged processes (CAP_SYS_RESOURCE
), but theHARD_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 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_zsingle_quotesz_s
mq_msgsize
setting when the queue is created with a call to mq_open(3) whereattr
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
. Ifmsgsize_default
exceedsmsgsize_max
, a new queue_zsingle_quotesz_s defaultmq_msgsize
value is capped to themsgsize_max
limit. Up until Linux 2.6.28, the defaultmq_msgsize
was 8192; from Linux 2.6.28 to Linux 3.4, the default was the value defined for themsgsize_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 formsgsize_max
is 8192 bytes. The minimum value is 128 (8192 in kernels before 2.6.28). The upper limit formsgsize_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, theHARD_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 thequeues_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_zsingle_quotesz_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 isSIGEV_NONE
; and 2 isSIGEV_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
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.
BUGS
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.
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), namespaces(7)
COLOPHON
This page is part of release 5.04 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
https://www.kernel.org/doc/man−pages/.
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