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 (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.
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)
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:
/proc/sys/fs/mqueue/msg_maxThis 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.
/proc/sys/fs/mqueue/msgsize_maxThis 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).
/proc/sys/fs/mqueue/queues_maxThis 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.
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).
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:
QSIZENumber of bytes of data in all messages in the queue.
NOTIFY_PIDIf 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.
NOTIFYNotification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2 is SIGEV_THREAD.
SIGNOSignal number to be used for SIGEV_SIGNAL.
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.
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)
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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t Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |