| mq_overview(7) | Miscellaneous Information Manual | mq_overview(7) |
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:
POSIX message queues have been supported since Linux 2.6.6. glibc support has been provided since glibc 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:
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:
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 (Linux 2.6.26) support the use of access control lists (ACLs) for POSIX message queues.
Since Linux 3.5 to Linux 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 Linux 3.5.x to Linux 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)
| 2023-02-05 | Linux man-pages 6.03 |