C Programming/Introduction

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Objective

Understand and differentiate between hardware and software.
Understand how the computer executes software.
Understand the concept of compiling and compiler
Set up and compile a C program
Gain a small taste of C: Create your very first C program and run it

Hardware & Software

A computer's system unit, also commonly known as the computer case, contains a CPU (Central Processing Unit) while it usually has a monitor, keyboard, and a mouse connected to it. The CPU does all of the the processing and is an important part of the computer. Without it, the computer wouldn't even be able to run. The monitor displays graphics on a screen, which is easy for the user to see. The monitor acts as visual output. The keyboard and the mouse act as user input. The computer uses this input for many things and you'll most likely get some kind of response because of it.

The computer uses all of this equipment to do its job. This equipment, called hardware, isn't the only thing that makes a computer run. With all of the hardware something else is still needed to bring the computer to life. The stuff that actually makes the computer do something is called software. Unlike hardware, software cannot be manipulated directly by the user. Usually, the user needs to use the computer to interact with the software.

The CPU processes the software and executes it appropriately. Software is made up of a lot of instructions which the CPU executes. We don't need to go in depth about the instructions, since they vary between CPU's. The important thing is that we know that there are instructions.

It is now understood that, if we want the computer to perform a task, we must first get the instructions first. How? We simply write them down and then turn them into executable code that the CPU can use. Just remember this, a computer program is a set of instructions (a "recipe") that defines the task to be performed.

Programming and programming language

Programming is the method we use to make the computer do what we want. Contrary to common belief, computers are not smart as they need to be told how to do even the simplest things down to the tiniest detail. Programming is the act of instructing the computer how to do something.

A programming language is the way we communicate with the computer. It's the same concept as a spoken language. When you talk to another person, you use a set of symbols (words) with a meaning you both know. When programming, you use a set of symbols that a program called a compiler knows. The compiler then translates it into a series of instructions for the processor.

Program Execution

Machine code is the computer's true language. Some would call it unreadable by human standards. However, it is digital poetry to the trained. Back when computer science was still in its infancy, all programs were written in machine language. Before screens and printers existed, the programmer laboriously entered into the computer the machine code for the program and then fed each instruction via some external device, such as punch cards or switches. This task of writing programs was extremely slow and prone to errors.

When a program is executed, the computer reads each instruction and performs the task associated with that instruction. Since each computer can vary in the type and amount of instructions it has, we won't focus too much on this topic. The important thing to know is C's ability to be cross-platform when creating code. This portability was the key to C's success.

Compiling and compiler

Machine code is basically a set of instructions that the computer can execute. Computers use binary as the format for storing information. Each number can either be a 1 or a 0; which could be represented as on and off. Thankfully, a new method of writing programs has replaced machine code; this is called assembly. Although this is pretty consistent to writing binary, assembly still gives the program some edge, leading to less wasted time. Still, coding in assembly was slow and error-prone. Higher-level programming languages started to appear. These made it easier for the programmer to write programs, efficiently and quickly. Some of these languages convert their code to assembly, which will come in handy later in the course. C, unlike a lot of new programming languages, converts its code to assembly where a new executable file is created.

A compiler is a piece of software which takes a number of files of source code (C code in this case) and converts into an executable file which you can run on your computer. Note that not every language is compiled - some are "interpreted" (read line-by-line as the code is executed), and some are a mix, using something called "bitcode" or "bytecode". The executable file produced by a compiler is in a language called machine code - normally a compiler also converts the code into assembly in-between, while trying to "optimize" it (make it faster and take up less RAM).

Understanding C

C is widely considered the lowest-level programming language, after assembly language and machine code. What this means is that C focuses much more on operations that work out exactly as they look - the logic used in the language is very similar to what the computer does. This differs from higher-level languages where list manipulation might be an option, whereas in C, you specify exact operations over a range of computer memory addresses to manipulate data in a specific way, which is known as memory management. This can either make things more or less intuitive for a person - sometimes you get weird results due to data types, and other times it can make processes easier. Since resource acquisition is one of the most common tasks in programming, any programmer with knowledge of memory management has an advantage.

Generally, however, C is viewed as being more difficult as you puzzle through the correct way to do certain things, and make sure you avoid dangerous syntax errors. However, knowing C is reward enough for the effort of learning the language - it is useful, flexible, and above all - very, very fast.

Most modern languages have a C-like syntax, even if they aren't based on C.

Setting up to program in C

C compiler

Linux or Macintosh computer has a built-in C compiler with them. Linux comes with GCC (GNU Compiler Collection - previously named GNU C Compiler), whereas MacOSX comes with Clang, a compiler that acts as a frontend to the LLVM compiler. For this course, we will use the GNU compiler.

Apple provides for MacOS the Xcode software, but it takes up a lot of disk space, so if you don't have a lot to spare, consider alternatives. Students working with MacOS for this course can use their built-in compiler Clang instead or install GCC on their Macintosh systems. To install GCC on a Macintosh system, all you need to do is download and install Xcode.

If you use Windows, installing a compiler can be a hassle. It is a good idea to install a development environment on Windows too - and it may come with a compiler attached to it!

For both OSes, I would recommend using Sublime Text (Sublime Text 3) (note that Sublime Text doesn't allow compilation within the application itself). Emacs is also a good choice, as this is one of the world's most flexible and universally compatible text editor.

The terminal

The terminal is the main way a programmer interacts with a computer. On MacOSX, it is an app under the name "Terminal", and is also called "Terminal" on many Linux distros. On Windows, you can use "Command Prompt". Upon opening, you will see a prompt to enter a command. To enter a command, type in to the command and hit the Enter key. An important thing to take note of is the filesystem. Your computer only normally looks in the current directory for needed files. To change the current directory, you can use the cd command on MacOSX or Linux. You can use cd --help to get more info on doing this.

We will go over the commands that we will be using in the first lesson. For the moment, simply open a terminal and enter "gcc" and press return. You should get a message that says "no input files" at the end. If so, your system have correctly been installed the gcc compiler.

Compiling programs

The way this is done will depend on how you are developing. If you are using a development environment then it may offer a way to compile within the app. However, it is impossible to cover all of this - just note that the option will normally fall under "Build" or "Compile". The command for compiling from the command line on MacOSX or Linux, however, can be covered. On Linux, simply use the gcc command, with the names of source files that are to be compiled into a single executable, and specify an output with the -o option (you should probably use a .o ending for this file on MacOSX, or .elf or .o on Linux). On MacOSX, you can use either gcc or clang to use the Clang compiler, as gcc is automatically symlinked to clang - meaning that both will actually execute Clang. The options are almost exactly the same for the two compilers, and the differences irrelevant for most purposes. It is a good idea to run the command gcc --help so that you can see a list of compiler options. You can also look online for more information.

C IDE

Beside using a C compiler like GCC to compile your program on the terminal, you can use an IDE, which is basically a text editor with one or several compilers built-into it, along with other GUI setup which helps you build your program easier, instead of typing the command like in the terminal. One candidate IDE for C starter is Dev-C++. However, using an IDE for a new starter is not recommended as it's better for any new commer to approach a new programming language from the CLI level, which is also the target of this course.

Beside installing C compiler along with its tool on your local machine, you can use the online C IDE which are totally free. Some famous free online C IDE are:

Syntax in C

The syntax of a programming language is a set of rules specifying the way(s) in which operations may be specified to be carried out by a computer. They restrict the ways in which one may specify calculation, and make sure that there can be no ambiguities in the way to interpret a given snippet of code.

Just like other programming languages, C defines a set of syntax to be followed so that the program and be compiled and executed successfully. The "must-have" syntax in every C program is ";" (the semicolon). Each block of any set of operations to be performed MUST end with a semicolon. Normally what happens is you will have some declaration or initialization or function call or assignment, and then before you can do your next set of calculations to reach an end effect upon whatever you want to change next, you will use a semicolon to break up the blocks. This may sound confusing, but when to use one is quite intuitive if you think about it as going after any chunk of grouped operations.

Let's head to the very first C program to have the first impression of their syntax.

A Taste of C

Here is the classic Hello World program written in C.

#include <stdio.h>

int main(void)
{
    printf("Hello, World!\n");
    return 0;
}

All C programs have the #include macro, the most common being #include <stdio.h>. What this macro really does is to tell the compiler, "Hey! Before you compile my program, just copy and paste the contents of stdio.h where I am!". This is used to include the contents of standard C libraries.

In C, code is cut up into little blocks of code called functions. Unless declared otherwise, the function main will be called first when the program starts. The center of any C program is the function main(). It is where the CPU first starts executing. Anything that needs to be executed by the program must be called/done by main(), or one of the functions it calls.

Don't worry about the words int and void, they are special sizes that will be discussed in the later lesson.

Next thing to be noticed from this program is the curly brackets ({}), as a function starts and ends with them.

The printf() function, which is arguably the most used function in C, is used to display formatted text.

At the heart of our program is an instruction to write "Hello, World!" (followed by a newline character "\n") to standard out, which is usually the console. So what exactly is happening here ? First, we call, or invoke, the function printf(). We put the arguments we wish to pass to printf() within the parenthesis. In the code above, we only pass a single argument, the string "Hello, World!\n". What's a string? Don't worry, that and more will be explained in the following lessons.

On the next line, we have return 0. When the computer encounters return 0, it immediately exits the function, meaning nothing past it will be executed, and returns the integer 0 to the caller of the function (in this case, the OS). When a non-zero integer is returned to the OS, it is usually a sign that an error has occurred somewhere in the program.

You may have noticed the semi-colon (;) at the end of printf("Hello, World!\n") and return 0. These lines of code are called statements, and in C, statements are always terminated with a ;. If you do not terminate your statements, the computer will not know where the statement ends and the next one begins.

Try writing the source code above to a file. Save the file (remember to save it with the extension .c if you're using a plain-text editor) then compile it.

Now we can compile the program using gcc within a terminal. Navigate to the directory where your file is saved and enter "gcc hello.c" (As we use GCC in this course). If all went well, you should see no messages at all. If you do, the compiler will tell you what type of error it encountered or what line the error is located in your program.

Assuming that all went well, you should see a new file called "a.out" in your working directory. This file is your first application binary! To run the program, simply type "./a.out". You should see "Hello World" printed into the command line.

You can now use standard UNIX commands to re-name your binary to something like "hello" instead of "a.out".

Congratulations! You've now just executed your first program that you wrote!

Comments

Comments, which play an important role in C programs, give details and explanations for the program reader about the purpose of the code. Developers can give comments for functions, variables and blocks of code. (We will discuss about functions and variables in the later lessons).

Two forward-slashes next to each other (and outside a string literal - explained later with the char data type) specify a line comment. A line comment tells the compiler to ignore the rest of the line. This allows a programmer to write little notes about their code. It is recommended that you comment your code as MUCH as possible. There are also multi-line comments. These start at any forward-slash-then-asterisk ("/*") and last until the next asterisk-then-forward-slash ("*/").

int a = 4; // This is a comment for variable a

/*
  This is a comment for function square()
*/
int square(int n)
{
     int ans = n * n;
     return ans;
}

Assignments

Name one piece of hardware that acts as user input.
Name one piece of hardware that acts as user output.
What is a computer program made up of?
What is the computer's true language?
What is the lowest-level, platform-dependent language called?
Is C compiled or assembled?
Were you able to compile the example in this lesson? If so, what happened?

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