Regular files
One process writes data into a file for another process to read [open,read,write].
Mmap'ed files
Processes map a file into their address spaces as a linear array. From what I have read, the resulting performance is usually lousy [mmap].
- BSD IPC
sockets
There are several varieties of sockets -- the two main families of interest are called UNIX sockets and INET sockets. In the INET family there are TCP (transmission control protocol) and UDP (user datagram protocol) sockets. UNIX and INET/TCP sockets are reliable byte streams; any data that you send via these sockets is guaranteed to to be reassembled at the destination in the proper order. To the user, UNIX and INET/TCP sockets look just like files: you send and receive data with the read() and write() system calls. INET/UDP sockets are unreliable and connectionless but preserve message boundaries. Data that is sent via INET/UDP sockets is not guaranteed to arrive at all. If it does arrive, the entire message arrives at once (messages are limited to about 8 KB). INET/UDP packets may also be duplicated and arrive out of order. Since UDP is connectionless, it is possible to send broadcast messages to all machines on a network. [socket,socketpair]
pipes
A pipe is a pair of connected UNIX sockets set up so that one process writes to the pipe and the other reads from it -- it's unidirectional [pipe]
- System V IPC
Shared memory segments
Shared memory segments allow different processes to share a common segment of physical memory. This is an exception to the rule that processes are not allowed to peek in on one another's address space. [shmget]
Semaphores
Semaphores are normally used in conjunction with shared memory and are similar in spirit to mutexes (described below). Basically, you use semaphores to serialize access to a piece of shared memory. After reading the rest of this document, you'll understand why this is necessary. [semget]
Message queues
Message queues allow processes to send small messages (16 KB maximum, usually) to one another. [msgget]
Earlier this semester, Anu gave a talk on PVM. PVM is based on IPC mechanisms; e.g., it uses sockets to communicate between processes on different machines across a network. On multiprocessor machines, PVM can be configured to use the System V IPCs to achieve communication between processes on different CPUs. One nice side effect of using these standard mechanisms is that PVM code is fairly portable between UNIX platforms.
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