POSIX Message Queues: Two Implementations

As described in the "Interprocess Communication (IPC)" chapter of the System Architecture guide, QNX Neutrino supports POSIX message queues. The OS includes two managers for message queues:

The traditional implementation.
An alternate implementation.

The mq implementation uses a queue within the kernel to buffer the messages and eliminates the (context-switching) overheads of using an external server (i.e., mqueue) in each message-queue operation, thus improving performance.


When you create a queue with mq, 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. (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 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. 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 common.mk 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 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