Managing POSIX Message Queues

QNX Neutrino has traditionally managed POSIX message queues using the mqueue server. Starting with release 6.3.0, QNX Neutrino implements an alternate POSIX message queue manager, the mq server. This implementation uses the kernel's asynchronous messaging facility to buffer the messages within the kernel itself, and eliminates the (context-switching) overheads of using an external server (i.e. mqueue) in each message-queue operation, thus greatly improving POSIX Message Queues performance.

The mq manager implements POSIX 1003.1b message queues. When you create a queue, it appears in the pathname space under /dev/mq. Note that it's different from the pathname space for mqueue - the queue appears in the pathname space under /dev/mqueue.

Note: You can change this directory to union over the directory exported by the mqueue server by using the mq -N/dev/mqueue option, but we don't recommend this, because it may cause some user-namespace confusion.

Although the mq server isn't involved in each mq_send()/mq_receive()/mq_notify() operation, the server is necessary to maintain the queue names and create the corresponding kernel message queues. You can also use ls and rm for administrative purposes, but you can't manipulate the queue contents by using shell utilities.

The following special client functions communicate with mqueue or mq:

Open or create a queue
Close a queue
Remove a message queue
Get attributes on a queue
Set attributes on a queue
Request notification when a message arrives
Send a message
Receive a message

For more information about these functions, see the QNX Neutrino C Library Reference.

By default, the implementation of the mq_*() routines in libc is the traditional style, using the mqueue server to broker each transaction.

In order to use the mq implementation, you must link your application(s) against the libmq library. In a manual build, specify the -l mq option; in automatic/recursive builds, use this setting in your file:

LIBS += mq

Note: When relinking applications to use the alternative implementation, be sure to change all affected components. We require such explicit intervention to use the alternate implementation because of potential incompatibilities if your code isn't strictly conforming to POSIX.

Here are some other differences between these two servers:

The following table summarizes the main differences between the alternate implementations:

Style Server Library Pathname mqd_t implementation Qnet
Traditional mqueue libc /dev/mqueue/ int (file descriptor) Yes
Alternative mq libmq /dev/mq/ int (internal index) No