Section (7) mq_overview
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
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
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,
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
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
configuration option. This option is enabled by
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
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_maxmsgsetting when the queue is created with a call to mq_open(3) where
attris 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 default
mq_maxmsgvalue will be the smaller of
msg_max. Up until Linux 2.6.28, the default
mq_maxmsgwas 10; from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
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_maxmsgargument given to mq_open(3). The default value for
msg_maxis 10. The minimum value is 1 (10 in kernels before 2.6.28). The upper limit is
msg_maxlimit is ignored for privileged processes (
CAP_SYS_RESOURCE), but the
HARD_MSGMAXceiling is nevertheless imposed.
The definition of
HARD_MSGMAXhas 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_msgsizesetting when the queue is created with a call to mq_open(3) where
attris specified as NULL. The default value for this file is 8192 (bytes). The minimum and maximum are as for
msgsize_max, a new queue_zsingle_quotesz_s default
mq_msgsizevalue is capped to the
msgsize_maxlimit. Up until Linux 2.6.28, the default
mq_msgsizewas 8192; from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
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_msgsizeargument given to mq_open(3). The default value for
msgsize_maxis 8192 bytes. The minimum value is 128 (8192 in kernels before 2.6.28). The upper limit for
msgsize_maxhas varied across kernel versions:
Before Linux 2.6.28, the upper limit is
From Linux 2.6.28 to 3.4, the limit is 1,048,576.
Since Linux 3.5, the limit is 16,777,216 (
msgsize_maxlimit is ignored for privileged process (
CAP_SYS_RESOURCE), but, since Linux 3.5, the
HARD_MSGSIZEMAXceiling is enforced for privileged processes.
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_maxis 256. No ceiling is imposed on the
queues_maxlimit; privileged processes (
CAP_SYS_RESOURCE) can exceed the limit (but see BUGS).
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.
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:
Number of bytes of data in all messages in the queue (but see BUGS).
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.
Notification method: 0 is
SIGEV_SIGNAL; 1 is
SIGEV_NONE; and 2 is
Signal number to be used for
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.
For a discussion of the interaction of POSIX message queue objects and IPC namespaces, see ipc_namespaces(7).
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.
In Linux versions 3.5 to 3.14, the kernel imposed a
ceiling of 1024 (
HARD_QUEUESMAX) on the value to which the
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.
An example of the use of various message queue functions is shown in mq_notify(3).
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Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com>
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