Software Tools: File Copying

The book begins with a simple function, getc() that reads a character and then returns that character as its return code. It doesn’t matter where it reads the character from. Consider it an “input stream”.

Next it introduces another simple function, putc() that takes a character as a parameter and outputs it. Again, the output destination is unimportant. Consider it an “output stream.”

So we have two trivial functions that perform trivial work one character at a time. What good is that? Well, if we put them together into a single function, we can take input from anywhere and output it anywhere. You may recognize this function as cat on a Linux system.

The only thing I have to worry about in this program is how to detect the end of the file. To solve that problem we choose an end-of-file character that we will call EOF. The book introduces a programming language, Ratfor, a Fortran pre-processor, that resembles C. Here is our function in Ratfor:

# copy - copy input characters to output

integer getc
integer c

while (getc(c) != EOF)
    putc(c)
stop
end

It uses integer values rather than chars to keep data types simple for the example code. It also uses indentation rather than curly braces or BEGIN/END statements to mark off code statements. And EOF is a symbolic constant to keep the code readable rather than using a magic number.

All of these are helpful practices for making code more readable. Martin Fowler’s quote, “Any fool can write code that a computer can understand. Good programmers write code that humans can understand.” came decades later!

Also consider that this kind of function is the basic building block of the Unix (and Linux and BSD and…) operating system. Take some input (any input) and put it on the output (any output). With it I can read a file from a disk and print it to the screen or to a printer. This is why any file or device on Linux is defined as file. It keeps the tools consistent and simple.

One tiny little program. It does one thing and does it well. It is an ideal software tool. With a collection of similarly designed software tools, we’ll be able to accomplish powerful work.

What are some other simple tools that you use?