Learning the C Programming Language

Part 1: hello, world

I’ve started to learn C. There’s a number of reasons for this. First, it was the ‘proper’ programming language when I was a kid. Second, I’ve been learning quite a bit of Go recently, and just about every other page on the excellent Go Blog has a sentence that starts with “In C…”.¹

Third, I’ve started a course on Data Structures and Algorithms on Coursera, inspired by the ever-inspirational Denise Yu. The course only accepts submissions in four languages: Python, Java, C++ and C.² Going through that list my mind went “Basically Ruby with bleaugh indentation, bleaugh Java bleaugh, sounds really hard, sounds hard.” So I went with ‘sounds hard’.

I thought I’d try and capture my process of learning C as it might be useful to others in a similar position - i.e. no computer science background but know how to program in Ruby and JavaScript. I’ll be approaching this in a series of posts, most of which will be following the loose structure of a presentation I gave on C.³

Background

C was invented by Dennis Richie at Bell labs in the 1970s in order to write UNIX.⁴ He needed a language that provided sufficient abstraction to program quickly and efficiently, while at the same time being able to communicate directly with the computer’s memory addresses to allow a programmer to perform low level optimizations. It has been a remarkably popular language, being used to write other languages (Ruby is written in C, NodeJS wouldn’t work without libuv, written in C), and heavily influencing most modern programming languages (Java, Ruby, JavaScript, and most obviously, Go).

hello, world

Another of the ways C has influenced programming is through The C Programming Language by Richie and Brian Kernighan. The author’s intials gave their name to a style of formatting code, along with giving us the de facto standard for your first program: “hello, world”.

#include <stdio.h>

int main() {
    printf("hello, world\n");
}

Line 1: Include a file called stdio.h. It includes information about the functions in the C standard library that deal with I/O - input/output. In this case printing to the terminal.

Line 3: All C programs start with a function called main - this is the function that gets executed when the program is, well, executed. The arguments (of which we’re using none at all) are in the parentheses. The body of the function is in the curly braces. So far so JavaScript. For people who have never seen a typed language the int is a little surprising. All it’s telling us (and C) is that the return value for this function is an integer. We’ll talk about types later, but for now the int is almost working like def in Ruby or function in JavaScript - it’s a keyword declaring a function.

Line 4: The meat of the program. Here we’re calling a function called printf which has already been written for us as a part of the C standard library - this is why we did that #include at the beginning. We’re calling it with a single argument, a string literal inside double quotes, that just says “hello, world” with a newline character (\n) at the end.

At the end of the line we put a semi-colon to tell C that the line has finished.

Compiling and running

If you put all of that into a file called hello-world.c, save it, head to the terminal and type⁵

gcc hello-world.c

Then if you ls the same directory you should see a new file calleda.out.⁶ If you then run this with ./a.out, you’ll see hello, world. Mission accomplished.

gcc is the Gnu Compiler Collection,⁷ which will compile your C program into an executable that your computer can run. All this means is that instead of translating each line of your program into something your computer can understand as you run it, as with something like Ruby or JavaScript, we’re translating the whole program in one go before we run it.

a.out isn’t that informative, so in order to get a better filename we can pass a flag to GCC:

gcc hello-world.c -o hello-word

Which outputs to the file hello-world, which we can now run with ./hello-world

Success, we’re now all C programmers!

Why main returns an int

If you’ve run programs on the command line before you may be aware that you get exit codes with each program that runs. You may have even been (un)lucky enough to see something on the lines of Error: non-zero exit code. On a POSIX system, a process returns a number to the process that called it to let it know how things went - this is called the exit code. 0 is the good one, every other number is some flavour of ‘gone wrong’.

The default return value for main, if we don’t explicitly return a value, is 0. We can change this behaviour in our hello-world by returning an explicit value using the keyword return (very Ruby, so JavaScript).

#include <stdio.h>

int main() {
    printf("hello, world\n");
    return 1;
}

(don’t forget the semicolon!)

Experiment with different return values. Remember to recompile your program each time you do. You may be able to see the returned value in your terminal’s prompt. Otherwise you can echo out the last commands exit status with the command echo $?.