4061 Session 13 (2/27)

1. 4061 Session 13 (2/27)

2. Today • Pipes and FIFOs

3. 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”

4. Admin • Quiz 2: Nice!

5. Inter-Process Communication (IPC) • Techniques for the exchange of data between processes or threads

6. Why IPC?

7. 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

8. 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

9. Pipes: Conceptually

10. 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

11. Opening a Pipe • A process issues a command to create a pipe, and receives two file descriptors.

12. Forking • To communicate with something, fork.

13. Cleaning Up • Close unused descriptors.

14. 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

15. Details • Often implemented as a circular buffer

16. Shortcut Methods • popen and pclose

17. 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