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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'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).
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 (Linux 2.6.26) support the use of access control lists (ACLs) for
POSIX message queues.
BUGS
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.
EXAMPLES
An example of the use of various message queue functions is shown in mq_notify(3).
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)