CSC195, Class 54: Even More Fun with Processes

Overview:
* Questions from previous class?
* Changing stdin and stdout
* Detour: Command line parameters
* Executing other programs
* Exercise: cat grep sort cat

Notes:
* I imagine many of you are having a hard time right now,
  whether or not you knew Mr. Stefanov.  I know I am.
  Is there anything I can do to help?
* Email me extra credit summaries
* Questions on the exam?  Extended until Friday.
* Tomorrow: Evaluate the class.
* Friday: Meet at Dari Barn.

Questions on the exam?

Q: Do I care what version of binary search you do?
A: No.
Q: How much to cite?
A: Don't cite wp.  Cite code.
Q: How to cite web pages?
Smith, John (2003).  "How to detect memory leaks in C programs."
The University of Toronto.  Online document available at
http://www.utoronto.edu.ca/smithj/memory-leaks.html (last
modified 23 May 2003; last visited 25 May 2003).

Questions on yesterday's stuff?

What does wait do if you have no children?
It returns -1.
So, to wait for all of your children (when you've forgotten
  how many you have), repeatedly call wait until it returns -1.

One of the thigns we learned on Friday was how to create a
Unix-style pipe.
* What is a pipe?
  A communication mechanism.  You shove stuff in one end
  of the pipe and it comes out the other one.
* How do we create one?
  + Create an array of two integers
    int fd[2];
  + Call pipe with that as a parameter
    pipe(fd);
* Why don't we need to pass the address of fd?
  + We (almost) never pass the address of an array.
  + The array name is an address, so you're passing the
    address of the first element
* How do we use one?
  + Use read and write.
  + write(fd[1], buffer, amount-of-buffer-to-write);
  + read(fd[0], buffer, amount-of-buffer-to-read);

Interesting mismatch.  Sometimes we use integers and
write and read.  Sometimes we use FILE *s with printf
and scanf/getc.  Can we unify the two?

There is a way to convert file decriptors to FILE *s.  I'll
admit that I just forgot it.

More importantly, there's a way to make standard output and
standard input use different file descriptors.

We do so with dup2

  dup2(fd[0], STDIN_FILENO);
  dup2(fd[1], STDOUT_FILENO);

To save first

  dup2(STDIN_FILENO, SAVESTDIN);
  dup2(fd[0], STDIN_FILENO);
  ...
  dup2(SAVESTDIN, STDIN_FILENO);

Exercise:

  Parent:
  + Creates a pipe, forks a child.  
  Child:
  + Reidrects standard output to the pipe.
  + Prints to the pipe with printf
  Parent:
  + Redicrect standard input from the pipe.
  + Reads with getc.
  + Prints to standard output

  When that works, restore the parent's standard input