February 8, 2006

Coverage: [USP] Chapter 6

Last Editor Example from Last Class

graphically describe the situation after a text editor, which makes a backup of each files it opens, changes a file with a symbolic link to it
how can we change our editor so that a symbolic or hard link to a file it modifies always reference the new file?
- use O_WRONLY and O_TRUNC flags
  - causes same inode to be used

a process communication buffer
basic ingredient for IPC (interprocess communication) in UNIX
data written to filedes[1] can be read from filedes[0] using FIFO
created with pipe function
read blocks until something is written to the pipe
- can be used as a synchronization mechanism
- can be used to create a barrier
not very useful for a single process
only usable by a process, who calls fork and its descendants

grep cats pets > _text
wc -l < num_cats
grep cats pets | wc -l
ps -A | grep grep
traditionally, but perhaps non-intuitively, the command to the right of the pipe symbol (|) is the parent of that to the left of it in the code
blocking nature of read and write synchronize the processes
use pipe and dup2 functions in concert to setup a pipeline
- illustrated several graphic (octopus-looking) depictions of this process
graphic depiction of the changes to the file descriptor table as before and after various function calls

unifies many concepts we have learned
- process creation and image swapping
- input/output
- interprocess communication
why return only error conditions?

pipes are temporary, FIFOs area permanent (until you unlink)
useful when the processes communicating were not created with fork
created with mkfifo
- function call and UNIX command
- resulting file contains p as the character to left of its file permission list in ls -l output
beauty of UNIX: unified file access interface
- regular files
- FIFOs
- terminals
leads to a simple client-server model