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. 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.
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:
LINUX SPECIFIC DETAILS
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:
/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 and minimum value for msg_max is 10; 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.
/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 and minimum value for msgsize_max is 8192
bytes; the upper limit is INT_MAX (2147483647 on
Linux/86). This limit is ignored for privileged
processes (CAP_SYS_RESOURCE).
/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. 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 to
INT_MAX.
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 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 using the following commands:
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:
These fields are as follows:
-
QSIZE Number of bytes of data in all messages in
the queue.
NOTIFY_PID-
If this is non-zero, 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.
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
portable.
CONFORMING TO
POSIX.1-2001.
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.
EXAMPLE
An example of the use of various message queue functions
is shown in mq_notify(3).
SEE ALSO
getrlimit(2), mq_getsetattr(2), mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), poll(2), select(2), epoll(4)
t
Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
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