Homework for chapter 5: Interlude: Process API

1. Write a program that calls fork(). Before calling fork(), have the main
program access a variable and set its value to something. What value is the
variable in the child process? What happens to the variable when both the child
and the parent change the value of x?

2. Write a program that opens a new file with open() and then calls fork() to
create a new process. Can both the child and the parent access the file
descriptor returned by open()? What happens when they are writing to the file
concurrently?

3. Write another program using fork(). The child process should print "hello",
the parent process should print "goodbye". You should try to ensure that the
child process always prints first; can you do this without calling wait() in
the parent?

4. Write a program that calls fork() and then calls some form of exec() to run
/bin/ls. See if you can try all the variants of exec(), including execl(),
execle(), execlp(), execv(), execvp(), and execevP(). Why do you think there
are so many variants of the same basic call?

5. Now write a program that uses wait() to wait for the child process to finish
in the parent. What does wait() return? What happens if you use wait() in the
child()?

6. Write a slight modification of the previous program, this time using
waitpid() instead of wait(). When would waitpid() be useful?

7. Write a program that creates a child process, and then in the child close
standard output. What happens if the child calls printf() to print some output
after closing the descriptor?

8. Write a program that creates two children, and connects the standard output
of one to the standard input of the other using the pipe() system call.