46.10. Summary

System V message queues allow processes to communicate by exchanging messages consisting of a numeric type plus a body containing arbitrary data. The distinguishing features of message queues are that message boundaries are preserved and that the receiver(s) can select messages by type, rather than reading messages in first-in, first-out order.

Various factors led us to conclude that other IPC mechanisms are usually preferable to System V message queues. One major difficulty is that message queues are not referred to using file descriptors. This means that we can’t employ various alternative I/O models with message queues; in particular, it is complex to simultaneously monitor both message queues and file descriptors to see if I/O is possible. Furthermore, the fact that message queues are connectionless (i.e., not reference counted) makes it difficult for an application to know when a queue may be deleted safely.