DESCRIPTION<\/a>
NOTES<\/a>
BUGS<\/a>
EXAMPLES<\/a>
SEE ALSO<\/a>
COLOPHON<\/a> <\/p>\n
\n
NAME <\/a> <\/h2>\nmq_overview \u2212 overview of POSIX message queues<\/p>\n
DESCRIPTION <\/a> <\/h2>\nPOSIX 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<\/b>(2), msgsnd<\/b>(2), msgrcv<\/b>(2), etc.), but provides similar functionality.<\/p>\n
Message queues are created and opened using mq_open<\/b>(3); this function returns a message queue descriptor<\/i> (mqd_t<\/i>), 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<\/i>; that is, a null-terminated string of up to NAME_MAX<\/b> (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<\/b>(3).<\/p>\n
Messages are transferred to and from a queue using mq_send<\/b>(3) and mq_receive<\/b>(3). When a process has finished using the queue, it closes it using mq_close<\/b>(3), and when the queue is no longer required, it can be deleted using mq_unlink<\/b>(3). Queue attributes can be retrieved and (in some cases) modified using mq_getattr<\/b>(3) and mq_setattr<\/b>(3). A process can request asynchronous notification of the arrival of a message on a previously empty queue using mq_notify<\/b>(3).<\/p>\n
A message queue descriptor is a reference to an open message queue description<\/i> (see open<\/b>(2)). After a fork<\/b>(2), a child inherits copies of its parent\u2019s 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<\/i>) that are associated with the open message queue description.<\/p>\n
Each message has an associated priority<\/i>, and messages are always delivered to the receiving process highest priority first. Message priorities range from 0 (low) to sysconf(_SC_MQ_PRIO_MAX)\u00a0–\u00a01<\/i> (high). On Linux, sysconf(_SC_MQ_PRIO_MAX)<\/i> 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.<\/p>\n
The remainder of this section describes some specific details of the Linux implementation of POSIX message queues.<\/p>\n
Library interfaces and system calls<\/b>
In most cases the mq_*<\/b>() 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:<\/p>\n
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<\/b>(2), msgsnd<\/b>(2), msgrcv<\/b>(2), etc.), but provides similar functionality.<\/p>\n
Message queues are created and opened using mq_open<\/b>(3); this function returns a message queue descriptor<\/i> (mqd_t<\/i>), 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<\/i>; that is, a null-terminated string of up to NAME_MAX<\/b> (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<\/b>(3).<\/p>\n
Messages are transferred to and from a queue using mq_send<\/b>(3) and mq_receive<\/b>(3). When a process has finished using the queue, it closes it using mq_close<\/b>(3), and when the queue is no longer required, it can be deleted using mq_unlink<\/b>(3). Queue attributes can be retrieved and (in some cases) modified using mq_getattr<\/b>(3) and mq_setattr<\/b>(3). A process can request asynchronous notification of the arrival of a message on a previously empty queue using mq_notify<\/b>(3).<\/p>\n
A message queue descriptor is a reference to an open message queue description<\/i> (see open<\/b>(2)). After a fork<\/b>(2), a child inherits copies of its parent\u2019s 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<\/i>) that are associated with the open message queue description.<\/p>\n
Each message has an associated priority<\/i>, and messages are always delivered to the receiving process highest priority first. Message priorities range from 0 (low) to sysconf(_SC_MQ_PRIO_MAX)\u00a0–\u00a01<\/i> (high). On Linux, sysconf(_SC_MQ_PRIO_MAX)<\/i> 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.<\/p>\n
The remainder of this section describes some specific details of the Linux implementation of POSIX message queues.<\/p>\n
Library interfaces and system calls<\/b>
In most cases the mq_*<\/b>() 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:<\/p>\n
![\"Image](\"grohtml-942571.png\")
<\/p>\n