17.10 Exercises

1. Add functions to implement system calls such as chown(), chmod(), and umask() to the file operations extension presented in C Code for chdir() and stat().
2. Write an input parser that prints a prompt if the input is a from a “terminal” device. You can use the isatty() function to tell if the input file is a terminal. (Hint: this function is usually expensive to call; try to call it just once.) The content of the prompt should come from a variable settable by awk-level code. You can write the prompt to standard error. However, for best results, open a new file descriptor (or file pointer) on /dev/tty and print the prompt there, in case standard error has been redirected.

   Why is standard error a better choice than standard output for writing the prompt? Which reading mechanism should you replace, the one to get a record, or the one to read raw bytes?

3. Write a wrapper script that provides an interface similar to ‘sed -i’ for the “inplace” extension presented in Enabling In-Place File Editing.