4061 Session 13 (2/27)

Today

• Pipes and FIFOs

Today’s Objectives• Understand the concept of IPC

• Understand the purpose of anonymous and named pipes

• Describe where file descriptors point after system calls to pipe, fork, and dup

• Write C code that allows parent and child processes to communicate with pipes or FIFOs– Specifically: write a “filter”

Admin

• Quiz 2: Nice!

Inter-Process Communication (IPC)

• Techniques for the exchange of data between processes or threads

Why IPC?

UNIX IPC

• Lots of ways to do it. We’ll talk about some of the classics in 4061.

• Today:– Pipes and FIFOs

• Later:– Signals, Sockets

Pipes

• Aka: anonymous pipes or unnamed pipes

• Very simple IPC

• Remember bash shell: |– Chain stdout of one process with stdin of

another process– E.g.: ps ax | grep java

Pipes: Conceptually

Pipes: Conceptually

• One-way channel for information– Write to the input end, read from the output

end

• Think of a pipe as a buffer in the operating system that you can read from and write to.– If you fill it up, the writer blocks– If it’s empty, the reader blocks

Opening a Pipe• A process issues a command to create a

pipe, and receives two file descriptors.

Forking• To communicate with something, fork.

Cleaning Up• Close unused descriptors.

Details

• What do the file descriptors point to?– Well, files...but a special type (in pipefs).– So we can still use file-oriented calls, such as

read, write, close, etc.– No mount point, so not visible with ls

• If you close one end of a pipe, you can’t reopen it

Details

• Often implemented as a circular buffer

Shortcut Methods

• popen and pclose

FIFOs (a.k.a. Named Pipes)

• (first in first out)

• File (of type FIFO) in the filesystem.– Access permissions

• Kernel structures are set up on open

• Multiple processes can write, only one can read

• Writes (smaller than the buffer) are atomic