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 (cf. 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
descriptors in the parent. Corresponding 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.1-2001 only
requires an implementation to support priorities in the range 0 to 31;
some implementations only provide 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
POSIX message queues have been supported on Linux since kernel 2.6.6.
Glibc support has been provided since version 2.3.4.
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.
The following interfaces can be used to limit the amount of kernel
memory consumed by POSIX message queues:
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_MAX:
(131072 / sizeof(void *)) (32768 on Linux/86). This limit is
ignored for privileged processes (CAP_SYS_RESOURCE), but the
HARD_MAX ceiling is nevertheless imposed.
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 is 1,048,576 (in kernels before 2.6.28, the upper
limit was INT_MAX; that is, 2,147,483,647 on Linux/86). This
limit is ignored for privileged processes (CAP_SYS_RESOURCE).
This file can be used to view and change the system-wide limit
on the number of message queues that can be created. Only
privileged processes (CAP_SYS_RESOURCE) can create new message
queues once this limit has been reached. The default value for
queues_max is 256; it can be changed to any value in the range 0
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 file system
On Linux, message queues are created in a virtual file system. (Other
implementations may also provide such a feature, but the details are
likely to differ.) This file system 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 file system 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.
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
SIGNO Signal number to be used for SIGEV_SIGNAL.
Polling message queue descriptors
On Linux, a message queue descriptor is actually a file descriptor, and
can be monitored using select(2), poll(2), or epoll(7). This is not
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.
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),
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