Class 54: Processes (3)

Held: Tuesday, 6 May 2003

Summary: We conclude our investigation of processes in Unix by considering how the new processes might execute other programs.

Related Pages:

• EBoard.
• peter.c, the program from yesterday's class.
• piper.c, the program from today's class.

Notes:

• What, if anything, can I do to help you get through this difficult time?
• Remember to send me a summary of your extra credit. If you want another point (limit is still 3), show up to Sylvia Techavalitpongse's talk on Internet Censorship in China, Thursday at 4:30 in 2413.
• Reminder: Tomorrow's class is reserved for evaluation. Friday's class will be held at Dari Barn.

Overview:

• Standard Input and Output.
• Detour: Command Line.
• Executing Other Prorams.
• Exercise.

Changing Standard Input and Output

• We can create new pipes and then make stdout write to those pipes or stdin read from those pipes
• Why would we want to do so?
• How:
  • Input: use dup2(pipexxx, STDIN_FILENO) : Output: use dup2(pipexxx, STDOUT_FILENO)

Detour: The Command Line

• Arguments argc and argv to main.
• Quick exercise: Write a program that prints out its command line parameters.

Running Other Programs

• Once you've forked a process (or even if you haven't, you can replace its code with another program.
• Lots and lots of variants. The primary one is execl.
• Usage: exec(programs, arg1, ..., argn, NULL);

Exercise

Create a program that takes two command-line parameters, the name of a file and a string. It should then open four processes and three pipes.

• The first process should open the named file, read its input, and print the output to the first pipe.
• The second process should execute grep to extract the lines with the given string.
• The third process should execute sort to put the lines in order.
• The fourth process should write the result to the screen.