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 (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.
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 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. Up until 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 in kernels
before 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 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. Up until 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 in kernels before 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 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).
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 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.
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).
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 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 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).
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)
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