From Linux Man Pages
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().
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 pri-
ority 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:
lB lB l l. Library interface System call mq_close(3) close(2) mq_getattr(3) mq_getsetattr(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)
LINUX SPECIFIC DETAILS
Versions
POSIX message queues have been supported on Linux since kernel 2.6.6. Glibc support has been provided since ver-
sion 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(), 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 msg-
size_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:
$ 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:
$ ls /dev/mqueue/mymq
QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
$ mount -t mqueue none /dev/mqueue
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).
RELATED
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)
CATEGORY