This type of IPC has several advantages because it is part of the file system. Most functions are performed with standard file system intrinsics that you are already familiar with. The cooperating processes find each other through an agreed-upon file name. Thus they don't have to determine each other's process ID.
The file system IPC facility uses a FIFO queue structure. Sending processes queue multiple messages that are stored until a receiver reads them, even across system shutdowns. Receiving processes are allowed to wait for messages on one or more empty queues. Messages are deleted from the queue as they are read.
The cooperating processes using IPC do not need to be related; that is, they don't need to be part of the same process tree. They can even be running on different machines on the same network.
There are several different ways to perform I/O. The :FILE command can be used to redirect the I/O to another disc device (local or remote), or to change the way in which the message file is accessed. The existing file system security features can also be used.
The heart of the file system IPC facility is the "message file." Message files reside partly in memory and partly on disc. MPE/iX uses the memory buffer part as much as possible, to achieve the best performance. The disc portion of the message file is used only as secondary storage in case the memory buffer part overflows. For many users of IPC, MPE/iX never accesses the disc portion of the message file.
