MQ_OVERVIEWSection: Linux Programmer's Manual (7)
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NAMEmq_overview - Overview of POSIX message queues
DESCRIPTIONPOSIX 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(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.
Library interfaces and system callsIn 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|
LINUX SPECIFIC DETAILS
VersionsPOSIX message queues have been supported on Linux since kernel 2.6.6. Glibc support has been provided since version 2.3.4.
Kernel configurationSupport for POSIX message queues is configurable via the CONFIG_POSIX_MQUEUE kernel configuration option. This option is enabled by default.
PersistencePOSIX message queues have kernel persistence: if not removed by mq_unlink(3), a message queue will exist until the system is shut down.
LinkingPrograms using the POSIX message queue API must be compiled with cc -lrt to link against the real-time library, librt.
/proc interfacesThe following interfaces can be used to limit the amount of kernel memory consumed by POSIX message queues:
- 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.
- 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 msgsize_max is 8192 bytes; the upper limit is INT_MAX (2147483647 on Linux/86). This limit is ignored for privileged processes (CAP_SYS_RESOURCE).
- 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 limitThe 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 systemOn 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/mqueueThe 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:
$ ls /dev/mqueue/mymq QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260 $ mount -t mqueue none /dev/mqueueThese fields are as follows:
- Number of bytes of data in all messages in the queue.
- 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.
- Notification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2 is SIGEV_THREAD.
- Signal number to be used for SIGEV_SIGNAL.
Polling message queue descriptorsOn 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.
NOTESSystem 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.
EXAMPLEAn example of the use of various message queue functions is shown in mq_notify(3).
SEE ALSOgetrlimit(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)
- LINUX SPECIFIC DETAILS
- CONFORMING TO
- SEE ALSO