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C Tutorial

{ Constant work in progress, mostly done but may still have some bugs. ~drummyfish }

This is a relatively quick C tutorial.

You should probably have at least some basic awareness of essential programming concepts before reading this (what's a programming language, source code, command line etc.). If you're as far as already somewhat knowing another language, this should be pretty easy to understand.

This tutorial focuses on teaching pure C, i.e. mostly just command line text-only programs. There is a small bonus that shows some very basics of doing graphics programming at the end, but bear in mind it's inevitable to learn step by step, as much as you want to start programming graphical games, you first HAVE TO learn the language itself well. Don't rush it. Trust this advice, it is sincere.

If you do two chapters a day (should take like half and hour), in a week you'll know some basic C.

Potentially supplemental articles to this tutorial are:

About C And Programming

C is

A programming language, i.e. a language that lets you express algorithms.
Compiled language (as opposed to interpreted), i.e. you have to compile the code you write (with compiler) in order to obtain a native executable program (a binary file that you can run directly).
Extremely fast and efficient.
Very widely supported and portable to almost anything.
Low level, i.e. there is relatively little abstraction and not many comfortable built-in functionality such as garbage collection, you have to write many things yourself, you will deal with pointers, endianness etc.
Imperative (based on sequences of commands), without object oriented programming.
Considered hard, but in certain ways it's simple, it lacks bloat and bullshit of "modern" languages which is an essential thing. It will take long to learn (don't worry, not nearly as long as learning a foreign language) but it's the most basic thing you should know if you want to create good software. You won't regret.
Not holding your hand, i.e. you may very easily "shoot yourself in your foot" and crash your program. This is the price for the language's power.
Very old, well established and tested by time.
Recommended by us for serious programs.

If you come from a language like Python or JavaScript, you may be shocked that C doesn't come with its own package manager, debugger or build system, it doesn't have modules, generics, garabage collection, OOP, hashmaps, dynamic lists, type inference and similar "modern" features. When you truly get into C, you'll find it's a good thing.

Programming in C works like this:

You write a C source code into a file.
You compile the file with a C compiler such as gcc (which is just a program that turns source code into a runnable program). This gives you the executable program.
You run the program, test it, see how it works and potentially get back to modifying the source code (step 1).

So, for writing the source code you'll need a text editor; any plain text editor will do but you should use some that can highlight C syntax -- this helps very much when programming and is practically a necessity. Ideal editor is vim but it's a bit difficult to learn so you can use something as simple as Gedit or Geany. We do NOT recommend using huge programming IDEs such as "VS Code" and whatnot. You definitely can NOT use an advanced document editor that works with rich text such as LibreOffice or that shit from Micro$oft, this won't work because it's not plain text.

Next you'll need a C compiler, the program that will turn your source code into a runnable program. We'll use the most commonly used one called gcc (you can try different ones such as clang or tcc if you want). If you're on a Unix-like system such as GNU/Linux (which you probably should), gcc is probably already installed. Open up a terminal and write gcc to see if it's installed -- if not, then install it (e.g. with sudo apt install build-essential if you're on a Debian-based system).

If you're extremely lazy, there are online web C compilers that work in a web browser (find them with a search engine). You can use these for quick experiments but note there are some limitations (e.g. not being able to work with files), and you should definitely know how to compile programs yourself.

Last thing: there are multiple standards of C. Here we will be covering C99, but this likely doesn't have to bother you at this point.

First Program

Let's quickly try to compile a tiny program to test everything and see how everything works in practice.

Open your text editor and paste this code:

/* simple C program! */

#include <stdio.h> // include IO library

int main(void)
{
  puts("It works.");

  return 0;
}

Save this file and name it program.c. Then open a terminal emulator (or an equivalent command line interface), locate yourself into the directory where you saved the file (e.g. cd somedirectory) and compile the program with the following command:

gcc -o program program.c

The program should compile and the executable program should appear in the directory. You can run it with

./program

And you should see

It works.

written in the command line.

Now let's see what the source code means:

/* simple C program! */ is so called block comment, it does nothing, it's here only for the humans that will read the source code. Such comments can be almost anywhere in the code. The comment starts at /* and ends with */.
// include IO library is another comment, but this is a line comment, it starts with // and ends with the end of line.
#include <stdio.h> tells the compiler we want to include a library named stdio (the weird syntax will be explained in the future). This is a standard library with input output functions, we need it to be able to use the function puts later on. We can include more libraries if we want to. These includes are almost always at the very top of the source code.
int main(void) is the start of the main program. What exactly this means will be explained later, for now just remember there has to be this function named main in basically every program -- inside it there are commands that will be executed when the program is run. Note that the curly brackets that follow ({ and }) denote the block of code that belongs to this function, so we need to write our commands between these brackets.
puts("It works."); is a "command" for printing text strings to the command line (it's a command from the stdio library included above). Why exactly this is written like this will be explained later, but for now notice the following. The command starts with its name (puts, for put string), then there are left and right brackets (( and )) between which there are arguments to the command, in our case there is one, the text string "It works.". Text strings have to be put between quotes ("), otherwise the compiler would think the words are other commands (the quotes are not part of the string itself, they won't be printed out). The command is terminated by ; -- all "normal" commands in C have to end with a semicolon.
return 0; is another "command", it basically tells the operating system that everything was terminated successfully (0 is a code for success). This command is an exception in that it doesn't have to have brackets (( and )). This doesn't have to bother us too much now, let's just remember this will always be the last command in our program.

Also notice how the source code is formatted, e.g. the indentation of code within the { and } brackets. White characters (spaces, new lines, tabs) are ignored by the compiler so we can theoretically write our program on a single line, but that would be unreadable. We use indentation, spaces and empty lines to format the code to be well readable.

To sum up let's see a general structure of a typical C program. You can just copy paste this for any new program and then just start writing commands in the main function.

#include <stdio.h> // include the I/O library
// more libraries can be included here

int main(void)
{
  // write commands here

  return 0; // always the last command
}

Variables, Arithmetic, Data Types

Programming is a lot like mathematics, we compute equations and transform numerical values into other values. You probably know in mathematics we use variables such as x or y to denote numerical values that can change (hence variables). In programming we also use variables -- here variable is a place in memory which has a name (and in this place there will be stored a value that can change over time).

We can create variables named x, y, myVariable or score and then store specific values (for now let's only consider numbers) into them. We can read from and write to these variables at any time. These variables physically reside in RAM, but we don't really care where exactly (at which address) they are located -- this is e.g. similar to houses, in common talk we normally say things like John's house or the pet store instead of house with address 3225.

Variable names can't start with a digit (and they can't be any of the keywords reserved by C). By convention they also shouldn't be all uppercase or start with uppercase (these are normally used for other things). Normally we name variables like this: myVariable or my_variable (pick one style, don't mix them).

In C as in other languages each variable has a certain data type; that is each variable has associated an information of what kind of data is stored in it. This can be e.g. a whole number, fraction, a text character, text string etc. Data types are a more complex topic that will be discussed later, for now we'll start with the most basic one, the integer type, in C called int. An int variable can store whole numbers in the range of at least -32768 to 32767 (but usually much more).

Let's see an example.

#include <stdio.h>

int main(void)
{
  int myVariable;

  myVariable = 5;

  printf("%d\n",myVariable);

  myVariable = 8;

  printf("%d\n",myVariable);
}

int myVariable; is so called variable declaration, it tells the compiler we are creating a new variable with the name myVariable and data type int. Variables can be created almost anywhere in the code (even outside the main function) but that's a topic for later.
myVariable = 5; is so called variable assignment, it stores a value 5 into variable named myVariable. IMPORTANT NOTE: the = does NOT signify mathematical equality but an assignment (equality in C is written as ==); when compiler encounters =, it simply takes the value on the right of it and writes it to the variable on the left side of it. Sometimes people confuse assignment with an equation that the compiler solves -- this is NOT the case, assignment is much more simple, it simply writes a value into variable. So x = x + 1; is a valid command even though mathematically it would be an equation without a solution.
printf("%d\n",myVariable); prints out the value currently stored in myVariable. Don't get scared by this complicated command, it will be explained later (once we learn about pointers). For now only know this prints the variable content.
myVariable = 8; assigns a new value to myVariable, overwriting the old.
printf("%d\n",myVariable); again prints the value in myVariable.

After compiling and running of the program you should see:

5
8

Last thing to learn is arithmetic operators. They're just normal math operators such as +, - and /. You can use these along with brackets (( and )) to create expressions. Expressions can contain variables and can themselves be used in many places where variables can be used (but not everywhere, e.g. on the left side of variable assignment, that would make no sense). E.g.:

#include <stdio.h>

int main(void)
{
  int heightCm = 175;
  int weightKg = 75;
  int bmi = (weightKg * 10000) / (heightCm * heightCm);

  printf("%d\n",bmi);
}

calculates and prints your BMI (body mass index).

Let's quickly mention how you can read and write values in C so that you can begin to experiment with your own small programs. You don't have to understand the following syntax as of yet, it will be explained later, now simply copy-paste the commands:

puts("hello");: Prints a text string with newline.
printf("hello");: Same as above but without newline.
printf("%d\n",x);: Prints the value of variable x with newline.
printf("%d ");: Same as above but without a newline.
scanf("%d",&x);: Read a number from input to the variable x. Note there has to be & in front of x.

Branches And Loops (If, While, For)

When creating algorithms, it's not enough to just write linear sequences of commands. Two things (called control structures) are very important to have in addition:

branches: Conditionally executing or skipping certain commands (e.g. if a user enters password we want to either log him in if the password was correct or write error if the password was incorrect). This is informally known as "if-then-else".
loops (also called iteration): Repeating certain commands given number of times or as long as some condition holds (e.g. when searching a text we repeatedly compare words one by one to the searched word until a match is found or end of text is reached).

Let's start with branches. In C the command for a branch is if. E.g.:

if (x > 10)
  puts("X is greater than 10.");

The syntax is given, we start with if, then brackets (( and )) follow inside which there is a condition, then a command or a block of multiple commands (inside { and }) follow. If the condition in brackets holds, the command (or block of commands) gets executed, otherwise it is skipped.

Optionally there may be an else branch which is gets executed only if the condition does NOT hold. It is denoted with the else keyword which is again followed by a command or a block of multiple commands. Branching may also be nested, i.e. branches may be inside other branches. For example:

if (x > 10)
  puts("X is greater than 10.");
else
{
  puts("X is not greater than 10.");

  if (x < 5)
    puts("And it is also smaller than 5.");
}

So if x is equal e.g. 3, the output will be:

X is not greater than 10.
And it is also smaller than 5.

About conditions in C: a condition is just an expression (variables/functions along with arithmetic operators). The expression is evaluated (computed) and the number that is obtained is interpreted as true or false like this: in C 0 (zero) means false, 1 (and everything else) means true. Even comparison operators like < and > are technically arithmetic, they compare numbers and yield either 1 or 0. Some operators commonly used in conditions are:

== (equals): yields 1 if the operands are equal, otherwise 0.
!= (not equal): yields 1 if the operands are NOT equal, otherwise 0.
< (less than): yields 1 if the first operand is smaller than the second, otherwise 0.
<=: yields 1 if the first operand is smaller or equal to the second, otherwise 0.
&& (logical AND): yields 1 if both operands are non-0, otherwise 0.
|| (logical OR): yields 1 if at least one operand is non-0, otherwise 0.
! (logical NOT): yields 1 if the operand is 0, otherwise 0.

E.g. an if statement starting as if (x == 5 || x == 10) will be true if x is either 5 or 10.

Next we have loops. There are multiple kinds of loops even though in theory it is enough to only have one kind of loop (there are multiple types out of convenience). The loops in C are:

while: Loop with condition at the beginning.
do while: Loop with condition at the end, not used so often so we'll ignore this one.
for: Loop executed a fixed number of times. This is a very common case, that's why there is a special loop for it.

The while loop is used when we want to repeat something without knowing in advance how many times we'll repeat it (e.g. searching a word in text). It starts with the while keyword, is followed by brackets with a condition inside (same as with branches) and finally a command or a block of commands to be looped. For instance:

while (x > y) // as long as x is greater than y
{
  printf("%d %d\n",x,y); // prints x and y

  x = x - 1; // decrease x by 1
  y = y * 2; // double y
}

puts("The loop ended.");

If x and y were to be equal 100 and 20 (respectively) before the loop is encountered, the output would be:

100 20
99 40
98 60
97 80
The loop ended.

The for loop is executed a fixed number of time, i.e. we use it when we know in advance how many time we want to repeat our commands. The syntax is a bit more complicated: it starts with the keywords for, then brackets (( and )) follow and then the command or a block of commands to be looped. The inside of the brackets consists of an initialization, condition and action separated by semicolon (;) -- don't worry, it is enough to just remember the structure. A for loop may look like this:

puts("Counting until 5...");

for (int i = 0; i < 5; ++i)
  printf("%d\n",i); // prints i

int i = 0 creates a new temporary variable named i (name normally used by convention) which is used as a counter, i.e. this variable starts at 0 and increases with each iteration (cycle), and it can be used inside the loop body (the repeated commands). i < 5 says the loop continues to repeat as long as i is smaller than 5 and ++i says that i is to be increased by 1 after each iteration (++i is basically just a shorthand for i = i + 1). The above code outputs:

Counting until 5...
0
1
2
3
4

IMPORTANT NOTE: in programming we count from 0, not from 1 (this is convenient e.g. in regards to pointers). So if we count to 5, we get 0, 1, 2, 3, 4. This is why i starts with value 0 and the end condition is i < 5 (not i <= 5).

Generally if we want to repeat the for loop N times, the format is for (int i = 0; i < N; ++i).

Any loop can be exited at any time with a special command called break. This is often used with so called infinite loop, a while loop that has 1 as a condition; recall that 1 means true, i.e. the loop condition always holds and the loop never ends. break allows us to place conditions in the middle of the loop and into multiple places. E.g.:

while (1) // infinite loop
{
  x = x - 1;

  if (x == 0)
    break; // this exits the loop!

  y = y / x;
}

The code above places a condition in the middle of an infinite loop to prevent division by zero in y = y / x.

Again, loops can be nested (we may have loops inside loops) and also loops can contain branches and vice versa.

Simple Game: Guess A Number

With what we've learned so far we can already make a simple game: guess a number. The computer thinks a random number in range 0 to 9 and the user has to guess it. The source code is following.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void)
{
  srand(clock()); // random seed

  while (1) // infinite loop
  {
    int randomNumber = rand() % 10;

    puts("I think a number. What is it?");

    int guess;

    scanf("%d",&guess); // read the guess

    getchar();

    if (guess == randomNumber)
      puts("You guessed it!");
    else
      printf("Wrong. The number was %d.\n",randomNumber);

    puts("Play on? [y/n]");

    char answer;

    scanf("%c",&answer); // read the answer

    if (answer == 'n')
      break;
  }

  puts("Bye.");

  return 0; // return success, always here
}

#include <stdlib.h>, #include <time.h>: we're including additional libraries because we need some specific functions from them (rand, srand, clock).
srand(clock());: don't mind this line too much, its purpose is to seed a pseudorandom number generator. Without doing this the game would always generate the same sequence of random numbers when run again.
while (1) is an infinite game loop -- it runs over and over, in each cycle we perform one game round. The loop can be exited with the break statement later on (if the user answers he doesn't want to continue playing).
int randomNumber = rand() % 10;: this line declares a variable named randomNumber and immediately assigns a value to it. The value is a random number from 0 to 9. This is achieved with a function rand (from the above included stdlib library) which returns a random number, and with the modulo (remainder after division) arithmetic operator (%) which ensures the number is in the correct range (less than 10).
int guess; creates another variable in which we'll store the user's guessed number.
scanf("%d",&guess); reads a number from the input to the variable named guess. Again, don't be bothered by the complicated structure of this command, for now just accept that this is how it's done.
getchar();: don't mind this line, it just discards a newline character read from the input.
if (guess == randomNumber) ...: this is a branch which checks if the user guess is equal to the generated random number. If so, a success message is printed out. If not, a fail message is printed out along with the secret number. Note we use the puts function for the first message as it only prints a text sting, while for the latter message we have to use printf, a more complex function, because that requires inserting a number into the printed string. More on these functions later.
char answer; declares a variable to store user's answer to a question of whether to play on. It is of char data type which can store a single text character.
scanf("%c",&answer); reads a single character from input to the answer variable.
if (answer == 'n') break; is a branch that exits the infinite loop with break statement if the answer entered was n (no).

Functions (Subprograms)

Functions are extremely important, no program besides the most primitive ones can be made without them (well, in theory any program can be created without functions, but in practice such programs would be extremely complicated and unreadable).

Function is a subprogram (in other languages functions are also called procedures or subroutines), i.e. it is code that solves some smaller subproblem that you can repeatedly invoke, for instance you may have a function for computing a square root, for encrypting data or for playing a sound from speakers. We have already met functions such as puts, printf or rand.

Functions are similar to but NOT the same as mathematical functions. Mathematical function (simply put) takes a number as input and outputs another number computed from the input number, and this output number depends only on the input number and nothing else. C functions can do this too but they can also do additional things such as modify variables in other parts of the program or make the computer do something (such as play a sound or display something on the screen) -- these are called side effects; things done besides computing an output number from an input number. For distinction mathematical functions are called pure functions and functions with side effects are called non-pure.

Why are function so important? Firstly they help us divide a big problem into small subproblems and make the code better organized and readable, but mainly they help us respect the DRY (Don't Repeat Yourself) principle -- this is extremely important in programming. Imagine you need to solve a quadratic equation in several parts of your program; you do NOT want to solve it in each place separately, you want to make a function that solves a quadratic equation and then only invoke (call) that function anywhere you need to solve your quadratic equation. This firstly saves space (source code will be shorter and compiled program will be smaller), but it also makes your program manageable and eliminates bugs -- imagine you find a better (e.g. faster) way to solving quadratic equations; without functions you'd have to go through the whole code and change the algorithm in each place separately which is impractical and increases the chance of making errors. With functions you only change the code in one place (in the function) and in any place where your code invokes (calls) this function the new better and updated version of the function will be used.

Besides writing programs that can be directly executed programmers write libraries -- collections of functions that can be used in other projects. We have already seen libraries such as stdio, standard input/output library, a standard (official, bundled with every C compiler) library for input/output (reading and printing values); stdio contains functions such as puts which is used to printing out text strings. Examples of other libraries are the standard math library containing function for e.g. computing sine, or SDL, a 3rd party multimedia library for such things as drawing to screen, playing sounds and handling keyboard and mouse input.

Let's see a simple example of a function that writes out a temperature in degrees of Celsius as well as in Kelvin:

#include <stdio.h>

void writeTemperature(int celsius)
{
  int kelvin = celsius + 273;
  printf("%d C (%d K)\n",celsius,kelvin);
}

int main(void)
{
  writeTemperature(-50);
  writeTemperature(0);
  writeTemperature(100);

  return 0;
}

The output is

-50 C (223 K)
0 C (273 K)
100 C (373 K)

Now imagine we decide we also want our temperatures in Fahrenheit. We can simply edit the code in writeTemperature function and the program will automatically be writing temperatures in the new way.

Let's see how to create and invoke functions. Creating a function in code is done between inclusion of libraries and the main function, and we formally call this defining a function. The function definition format is following:

RETURN_TYPE FUNCTION_NAME(FUNCTION_PARAMETERS)
{
  FUNCTION_BODY
}

RETURN_TYPE is the data type the function returns. A function may or may not return a certain value, just as the pure mathematical function do. This may for example be int, if the function returns an integer number. If the function doesn't return anything, this type is void.
FUNCTION_NAME is the name of the function, it follows the same rules as the names for variables.
FUNCTION_PARAMETERS specifies the input values of the function. The function can take any number of parameters (e.g. a function playBeep may take 0 arguments, sine function takes 1, logarithm may take two etc.). This list is comma-separated and each item consists of the parameter data type and name. If there are 0 parameters, there should be the word void inside the brackets, but compilers tolerate just having empty brackets.
FUNCTION_BODY are the commands executed by the function, just as we know them from the main function.

Let's see another function:

#include <stdio.h>

int power(int x, int n)
{
  int result = 1;

  for (int i = 0; i < n; ++i) // repeat n times
    result = result * x;

  return result;
}

int main(void)
{
  for (int i = 0; i < 5; ++i)
  {
    int powerOfTwo = power(2,i);
    printf("%d\n",powerOfTwo);
  }

  return 0;
}

The output is:

The function power takes two parameters: x and n, and returns x raised to the ns power. Note that unlike the first function we saw here the return type is int because this function does return a value. Notice the command return -- it is a special command that causes the function to terminate and return a specific value. In functions that return a value (their return type is not void) there has to be a return command. In function that return nothing there may or may not be one, and if there is, it has no value after it (return;);

Let's focus on how we invoke the function -- in programming we say we call the function. The function call in our code is power(2,i). If a function returns a value (return type is not void), its function call can be used in any expression, i.e. almost anywhere where we can use a variable or a numerical value -- just imagine the function computes a return value and this value is substituted to the place where we call the function. For example we can imagine the expression power(3,1) + power(3,0) as simply 3 + 1.

If a function returns nothing (return type is void), it can't be used in expressions, it is used "by itself"; e.g. playBeep();. (Function that do return a value can also be used like this -- their return value is in this case simply ignored.)

We call a function by writing its name (power), then adding brackets (( and )) and inside them we put arguments -- specific values that will substitute the corresponding parameters inside the function (here x will take the value 2 and n will take the current value of i). If the function takes no parameters (the function parameter list is void), we simply put nothing inside the brackets (e.g. playBeep(););

Here comes the nice thing: we can nest function calls. For example we can write x = power(3,power(2,1)); which will result in assigning the variable x the value of 9. Functions can also call other functions (even themselves, see recursion), but only those that have been defined before them in the source code (this can be fixed with so called forward declarations).

Notice that the main function we always have in our programs is also a function definition. The definition of this function is required for runnable programs, its name has to be main and it has to return int (an error code where 0 means no error). It can also take parameters but more on that later.

These is the most basic knowledge to have about C functions. Let's see one more example with some pecularities that aren't so important now, but will be later.

#include <stdio.h>

void writeFactors(int x) // writes divisors of x
{
  printf("factors of %d:\n",x);

  while (x > 1) // keep dividing x by its factors
  {
    for (int i = 2; i <= x; ++i) // search for a factor
      if (x % i == 0) // i divides x without remainder?
      {
        printf("  %d\n",i); // i is a factor, write it
        x = x / i; // divide x by i
        break; // exit the for loop
      }
  }
}

int readNumber(void)
{
  int number;

  puts("Please enter a number to factor (0 to quit).");
  scanf("%d",&number);

  return number;
}

int main(void)
{
  while (1) // infinite loop
  {
    int number = readNumber(); // <- function call

    if (number == 0) // 0 means quit
      break;

    writeFactors(number); // <- function call
  }

  return 0;
}

We have defined two functions: writeFactors and readNumber. writeFactors return no values but it has side effects (print text to the command line). readNumber takes no parameters but return a value; it prompts the user to enter a value and returns the read value.

Notice that inside writeFactors we modify its parameter x inside the function body -- this is okay, it won't affect the argument that was passed to this function (the number variable inside the main function won't change after this function call). x can be seen as a local variable of the function, i.e. a variable that's created inside this function and can only be used inside it -- when writeFactors is called inside main, a new local variable x is created inside writeFactors and the value of number is copied to it.

Another local variable is number -- it is a local variable both in main and in readNumber. Even though the names are the same, these are two different variables, each one is local to its respective function (modifying number inside readNumber won't affect number inside main and vice versa).

And a last thing: keep in mind that not every command you write in C program is a function call. E.g. control structures (if, while, ...) and special commands (return, break, ...) are not function calls.

More Details (Globals, Switch, Float, Forward Decls, Program Arguments, ...)

We've skipped a lot of details and small tricks for simplicity. Let's go over some of them. Many of the following things are so called syntactic sugar: convenient syntax shorthands for common operations.

Multiple variables can be defined and assigned like this:

int x = 1, y = 2, z;

The meaning should be clear, but let's mention that z doesn't generally have a defined value here -- it will have a value but you don't know what it is (this may differ between different computers and platforms). See undefined behavior.

The following is a shorthand for using operators:

x += 1;      // same as: x = x + 1;
x -= 10;     // same as: x = x - 1;
x *= x + 1;  // same as: x = x * (x + 1);
x++;         // same as: x = x + 1;
x--;         // same as: x = x - 1;
// etc.

The last two constructs are called incrementing and decrementing. This just means adding/subtracting 1.

In C there is a pretty unique operator called the ternary operator (ternary for having three operands). It can be used in expressions just as any other operators such as + or -. Its format is:

CONDITION ? VALUE1 : VALUE2

It evaluates the CONDITION and if it's true (non-0), this whole expression will have the value of VALUE1, otherwise its value will be VALUE2. It allows for not using so many ifs. For example instead of

if (x >= 10)
  x -= 10;
else
  x = 10;

we can write

x = x >= 10 ? x - 10 : 10;

Global variables: we can create variables even outside function bodies. Recall than variables inside functions are called local; variables outside functions are called global -- they can basically be accessed from anywhere and can sometimes be useful. For example:

#include <stdio.h>
#include <stdlib.h> // for rand()

int money = 0; // total money, global variable

void printMoney(void)
{
  printf("I currently have $%d.\n",money);
}

void playLottery(void)
{
  puts("I'm playing lottery.");

  money -= 10; // price of lottery ticket

  if (rand() % 5) // 1 in 5 chance
  {
    money += 100;
    puts("I've won!");
  }
  else
    puts("I've lost!");

  printMoney();
}

void work(void)
{
  puts("I'm going to work :(");

  money += 200; // salary

  printMoney();
}

int main()
{
  work();
  playLottery();
  work();
  playLottery();

  return 0;
}

In C programs you may encounter a switch statement -- it is a control structure similar to a branch if which can have more than two branches. It looks like this:

  switch (x)
  {
    case 0: puts("X is zero. Don't divide by it."); break;
    case 69: puts("X is 69, haha."); break;
    case 42: puts("X is 42, the answer to everything."); break;
    default: printf("I don't know anything about X."); break;
  }

Switch can only compare exact values, it can't e.g. check if a value is greater than something. Each branch starts with the keyword case, then the match value follows, then there is a colon (:) and the branch commands follow. IMPORTANT: there has to be the break; statement at the end of each case branch (we won't go into details). A special branch is the one starting with the word default that is executed if no case label was matched.

Let's also mention some additional data types we can use in programs:

char: A single text character such as 'a', 'G' or '_'. We can assign characters as char c = 'a'; (single characters are enclosed in apostrophes similarly to how text strings are inside quotes). We can read a character as c = getchar(); and print it as putchar(c);. Special characters that can be used are \n (newline) or \t (tab). Characters are in fact small numbers (usually with 256 possible values) and can be used basically anywhere a number can be used (for example we can compare characters, e.g. if (c < 'b') ...). Later we'll see characters are basic building blocks of text strings.
unsigned int: Integer that can only take positive values or 0 (i.e. no negative values). It can store higher positive values than normal int (which is called a signed int).
long: Big integer, takes more memory but can store number in the range of at least a few billion.
float and double: Floating point number (double is bigger and more precise than float) -- an approximation of real numbers, i.e. numbers with a fractional part such as 2.5 or 0.0001. You can print these numbers as printf("%lf\n",x); and read them as scanf("%f",&x);.

Here is a short example with the new data types:

#include <stdio.h>

int main(void)
{
  char c;
  float f;

  puts("Enter character.");
  c = getchar(); // read character

  puts("Enter float.");
  scanf("%f",&f);

  printf("Your character is :%c.\n",c);
  printf("Your float is %lf\n",f);
 
  float fSquared = f * f;
  int wholePart = f; // this can be done

  printf("It's square is %lf.\n",fSquared);
  printf("It's whole part is %d.\n",wholePart);

  return 0;
}

Notice mainly how we can assign a float value into the variable of int type (int wholePart = f;). This can be done even the other way around and with many other types. C can do automatic type conversions (casting), but of course, some information may be lost in this process (e.g. the fractional part).

In the section about functions we said a function can only call a function that has been defined before it in the source code -- this is because the compiler read the file from start to finish and if you call a function that hasn't been defined yet, it simply doesn't know what to call. But sometimes we need to call a function that will be defined later, e.g. in cases where two functions call each other (function A calls function B in its code but function B also calls function A). For this there exist so called forward declarations -- a forward declaration is informing that a function of certain name (and with certain parameters etc.) will be defined later in the code. Forward declaration look the same as a function definition, but it doesn't have a body (the part between { and }), instead it is terminated with a semicolon (;). Here is an example:

#include <stdio.h>

void printDecorated2(int x, int fancy); // forward declaration

void printDecorated1(int x, int fancy)
{
  putchar('~');

  if (fancy)
    printDecorated2(x,0); // would be error without f. decl. 
  else
    printf("%d",x);

  putchar('~');
}

void printDecorated2(int x, int fancy)
{
  putchar('>');

  if (fancy)
    printDecorated1(x,0);
  else
    printf("%d",x);

  putchar('<');
}

int main()
{
  printDecorated1(10,1);
  putchar('\n'); // newline
  printDecorated2(20,1);
}

which prints

~>10<~
>~20~<

The functions printDecorated1 and printDecorated2 call each other, so this is the case when we have to use a forward declaration of printDecorated2. Also note the condition if (fancy) which is the same thing as if (fancy != 0) (imagine fancy being 1 and 0 and about what the condition evaluates to in each case).

And one more important thing: our program as a whole can be passed parameters when it's executed, which inside the program we can access as so called command line arguments (also known as flags, switches etc.). This is important especially under Unix operating systems where we run programs from command line and where programs often work in non-interactive ways and are composed into bigger programs (similarly to how we compose small C functions into one big program); command line arguments are similar to arguments we pass to functions, they can inform our program to behave in certain way, for example to open a certain config file at start, to run in fullscreen mode, to print help and so on. When we compile our programs with the gcc compiler, e.g. like gcc -o myprogram myprogram.c, all the text after gcc are in fact arguments telling gcc which program to compile, how to compile it, how to name the output and so on. To allow our program to receive these arguments we add two parameters to the main function, one called argc (argument count) of integer type, saying how many arguments we get, and another called argv (argument vector) of pointer to a pointer to char type (please don't get scared), holding an array of strings (each argument will be of string type). Operating system will automatically fill these arguments in when our program is started. Here is a short example demonstrating this:

#include <stdio.h>

int main(int argc, char **argv)
{
  puts("You passed these arguments:");

  for (int i = 0; i < argc; ++i)
    printf("- \"%s\"\n",argv[i]);

  return 0;
}

If you compile this program and run it e.g. like

./program hello these are "my arguments"

The program will output:

You passed these arguments:
- "./program"
- "hello"
- "these"
- "are"
- "my arguments"

Things to notice here are following: we passed 4 arguments but got 5 -- the first argument is the path of our program itself, i.e. we will always get at least this argument. Then we also see that our arguments are separated by spaces, but if we put them into double quotes (like the last one), it will become just one argument, keeping the spaces (but not the quotes). For now this knowledge will suffice, you will most definitely encounter command line arguments in real programs -- now you know what they are.

Header Files, Libraries, Compilation/Building

So far we've only been writing programs into a single source code file (such as program.c). More complicated programs consist of multiple files and libraries -- we'll take a look at this now.

In C we normally deal with two types of source code files:

.c files: These files contain so called implementation of algorithms, i.e. code that translates into actual program instructions. These files are what's handed to the compiler.
.h files, or header files: These files typically contain declarations such as constants and function headers (but not their bodies, i.e. implementations).

When we have multiple source code files, we typically have pairs of .c and .h files. E.g. if there is a library called mathfunctions, it will consist of files mathfunctions.c and mathfunctions.h. The .h file will contain the function headers (in the same manner as with forward declarations) and constants such as pi. The .c file will then contain the implementations of all the functions declared in the .h file. But why do we do this?

Firstly .h files may serve as a nice documentation of the library for programmers: you can simply open the .h file and see all the functions the library offers without having to skim over thousands of lines of code. Secondly this is for how multiple source code files are compiled into a single executable program.

Suppose now we're compiling a single file named program.c as we've been doing until now. The compilation consists of several steps:

The compiler reads the file program.c and makes sense of it.
It then creates an intermediate file called program.o. This is called an object file and is a binary compiled file which however cannot yet be run because it is not linked -- in this code all memory addresses are relative and it doesn't yet contain the code from external libraries (e.g. the code of printf).
The compiler then runs a linker which takes the file program.o and the object files of libraries (such as the stdio library) and it puts them all together into the final executable file called program. This is called linking; the code from the libraries is copied to complete the code of our program and the memory addresses are settled to some specific values.

So realize that when the compiler is compiling our program (program.c), which contains function such as printf from a separate library, it doesn't have the code of these functions available -- this code is not in our file. Recall that if we want to call a function, it must have been defined before and so in order for us to be able to call printf, the compiler must know about it. This is why we include the stdio library at the top of our source code with #include <stdio.h> -- this basically copy-pastes the content of the header file of the stdio library to the top of our source code file. In this header there are forward declarations of functions such as printf, so the compiler now knows about them (it knows their name, what they return and what parameters they take) and we can call them.

Let's see a small example. We'll have the following files (all in the same directory).

library.h (the header file):

// Returns the square of n.
int square(int n);

library.c (the implementation file):

int square(int x)
{
  // function implementation
  return x * x;
}

program.c (main program):

#include <stdio.h>
#include "library.h"

int main(void)
{
  int n = square(5);

  printf("%d\n",n);

  return 0;
}

NOTE: "library.h" here is between double quotes, unlike <stdio.h>. This just says we specify an absolute path to the file as it's not in the directory where installed libraries go.

Now we will manually compile the library and the final program. First let's compile the library, in command line run:

gcc -c -o library.o library.c

The -c flag tells the compiler to only compile the file, i.e. only generate the object (.o) file without trying to link it. After this command a file library.o should appear. Next we compile the main program in the same way:

gcc -c -o program.o program.c

This will generate the file program.o. Note that during this process the compiler is working only with the program.c file, it doesn't know the code of the function square, but it knows this function exists, what it returns and what parameter it has thanks to us including the library header library.h with #include "library.h" (quotes are used instead of < and > to tell the compiler to look for the files in the current directory).

Now we have the file program.o in which the compiled main function resides and file library.o in which the compiled function square resides. We need to link them together. This is done like this:

gcc -o program program.o library.o

For linking we don't need to use any special flag, the compiler knows that if we give it several .o files, it is supposed to link them. The file program should appear that we can already run and it should print

This is the principle of compiling multiple C files (and it also allows for combining C with other languages). This process is normally automated, but you should know how it works. The systems that automate this action are called build systems, they are for example Make and Cmake. When using e.g. the Make system, the whole codebase can be built with a single command make in the command line.

Some programmers simplify this whole process further so that they don't even need a build system, e.g. with so called header-only libraries, but this is outside the scope of this tutorial.

As a bonus, let's see a few useful compiler flags:

-O1, -O2, -O3: Optimize for speed (higher number means better optimization). Adding -O3 normally instantly speeds up your program. This is recommended.
-Os: Optimize for size, the same as above but the compiler will try to make as small executable as possible.
-Wall -Wextra -pedantic: The compiler will write more warnings and will be more strict. This can help spot many bugs.
-c: Compile only (generate object files, do not link).
-g: Include debug symbols, this will be important for debugging.

Advanced Data Types And Variables (Structs, Arrays, Strings)

Until now we've encountered simple data types such as int, char or float. These identify values which can take single atomic values (e.g. numbers or text characters). Such data types are called primitive types.

Above these there exist compound data types (also complex or structured) which are composed of multiple primitive types. They are necessary for any advanced program.

The first compound type is a structure, or struct. It is a collection of several values of potentially different data types (primitive or compound). The following code shows how a struct can be created and used.

#include <stdio.h>

typedef struct
{
  char initial; // initial of name
  int weightKg;
  int heightCm;
} Human;

int bmi(Human human)
{
  return (human.weightKg * 10000) / (human.heightCm * human.heightCm);
}

int main(void)
{
  Human carl;

  carl.initial = 'C';
  carl.weightKg = 100;
  carl.heightCm = 180;

  if (bmi(carl) > 25)
    puts("Carl is fat.");

  return 0;
}

The part of the code starting with typedef struct creates a new data type that we call Human (one convention for data type names is to start them with an uppercase character). This data type is a structure consisting of three members, one of type char and two of type int. Inside the main function we create a variable carl which is of Human data type. Then we set the specific values -- we see that each member of the struct can be accessed using the dot character (.), e.g. carl.weightKg; this can be used just as any other variable. Then we see the type Human being used in the parameter list of the function bmi, just as any other type would be used.

What is this good for? Why don't we just create global variables such as carl_initial, carl_weightKg and carl_heightCm? In this simple case it might work just as well, but in a more complex code this would be burdening -- imagine we wanted to create 10 variables of type Human (john, becky, arnold, ...). We would have to painstakingly create 30 variables (3 for each person), the function bmi would have to take two parameters (height and weight) instead of one (human) and if we wanted to e.g. add more information about every human (such as hairLength), we would have to manually create another 10 variables and add one parameter to the function bmi, while with a struct we only add one member to the struct definition and create more variables of type Human.

Structs can be nested. So you may see things such as myHouse.groundFloor.livingRoom.ceilingHeight in C code.

Another extremely important compound type is array -- a sequence of items, all of which are of the same data type. Each array is specified with its length (number of items) and the data type of the items. We can have, for instance, an array of 10 ints, or an array of 235 Humans. The important thing is that we can index the array, i.e. we access the individual items of the array by their position, and this position can be specified with a variable. This allows for looping over array items and performing certain operations on each item. Demonstration code follows:

#include <stdio.h>
#include <math.h> // for sqrt()

int main(void)
{
  float vector[5];

  vector[0] = 1;
  vector[1] = 2.5;
  vector[2] = 0;
  vector[3] = 1.1;
  vector[4] = -405.054; 

  puts("The vector is:");

  for (int i = 0; i < 5; ++i)
    printf("%lf ",vector[i]);

  putchar('\n'); // newline

  /* compute vector length with
     pythagoren theorem: */

  float sum = 0;

  for (int i = 0; i < 5; ++i)
    sum += vector[i] * vector[i];

  printf("Vector length is: %lf\n",sqrt(sum));

  return 0;
}

We've included a new library called math.h so that we can use a function for square root (sqrt). (If you have trouble compiling the code, add -lm flag to the compile command.)

float vector[5]; is a declaration of an array of length 5 whose items are of type float. When compiler sees this, it creates a continuous area in memory long enough to store 5 numbers of float type, the numbers will reside here one after another.

After doing this, we can index the array with square brackets ([ and ]) like this: ARRAY_NAME[INDEX] where ARRAY_NAME is the name of the array (here vector) and INDEX is an expression that evaluates to integer, starting with 0 and going up to the vector length minus one (remember that programmers count from zero). So the first item of the array is at index 0, the second at index 1 etc. The index can be a numeric constant like 3, but also a variable or a whole expression such as x + 3 * myFunction(). Indexed array can be used just like any other variable, you can assign to it, you can use it in expressions etc. This is seen in the example. Trying to access an item beyond the array's bounds (e.g. vector[100]) will likely crash your program.

Especially important are the parts of code staring with for (int i = 0; i < 5; ++i): this is an iteration over the array. It's a very common pattern that we use whenever we need to perform some action with every item of the array.

Arrays can also be multidimensional, but we won't bothered with that right now.

Why are arrays so important? They allow us to work with great number of data, not just a handful of numeric variables. We can create an array of million structs and easily work with all of them thanks to indexing and loops, this would be practically impossible without arrays. Imagine e.g. a game of chess; it would be very silly to have 64 plain variables for each square of the board (squareA1, squareA2, ..., squareH8), it would be extremely difficult to work with such code. With an array we can represent the square as a single array, we can iterate over all the squares easily etc.

One more thing to mention about arrays is how they can be passed to functions. A function can have as a parameter an array of fixed or unknown length. There is also one exception with arrays as opposed to other types: if a function has an array as parameter and the function modifies this array, the array passed to the function (the argument) will be modified as well (we say that arrays are passed by reference while other types are passed by value). We know this wasn't the case with other parameters such as int -- for these the function makes a local copy that doesn't affect the argument passed to the function. The following example shows what's been said:

#include <stdio.h>

// prints an int array of lengt 10
void printArray10(int array[10])
{
  for (int i = 0; i < 10; ++i)
    printf("%d ",array[i]);
}

// prints an int array of arbitrary lengt
void printArrayN(int array[], int n)
{
  for (int i = 0; i < n; ++i)
    printf("%d ",array[i]);
}

// fills an array with numbers 0, 1, 2, ...
void fillArrayN(int array[], int n)
{
  for (int i = 0; i < n; ++i)
    array[i] = i;
}

int main(void)
{
  int array10[10];
  int array20[20];

  fillArrayN(array10,10);
  fillArrayN(array20,20);

  printArray10(array10);
  putchar('\n');
  printArrayN(array20,20);

  return 0;
}

The function printArray10 has a fixed length array as a parameter (int array[10]) while printArrayN takes as a parameter an array of unknown length (int array[]) plus one additional parameter to specify this length (so that the function knows how many items of the array it should print). The function printArray10 is important because it shows how a function can modify an array: when we call fillArrayN(array10,10); in the main function, the array array10 will be actually modified after when the function finishes (it will be filled with numbers 0, 1, 2, ...). This can't be done with other data types (though there is a trick involving pointers which we will learn later).

Now let's finally talk about text strings. We've already seen strings (such as "hello"), we know we can print them, but what are they really? A string is a data type, and from C's point of view strings are nothing but arrays of chars (text characters), i.e. sequences of chars in memory. In C every string has to end with a 0 char -- this is NOT '0' (whose ASCII value is 48) but the direct value 0 (remember that chars are really just numbers). The 0 char cannot be printed out, it is just a helper value to terminate strings. So to store a string "hello" in memory we need an array of length at least 6 -- one for each character plus one for the terminating 0. These types of string are called zero terminated strings (or C strings).

When we write a string such as "hello" in our source, the C compiler creates an array in memory for us and fills it with characters 'h', 'e', 'l', 'l', 'o', 0. In memory this may look like a sequence of numbers 104, 101, 108, 108 111, 0.

Why do we terminate strings with 0? Because functions that work with strings (such as puts or printf) don't know what length the string is. We can call puts("abc"); or puts("abcdefghijk"); -- the string passed to puts has different length in each case, and the function doesn't know this length. But thanks to these strings ending with 0, the function can compute the length, simply by counting characters from the beginning until it finds 0 (or more efficiently it simply prints characters until it finds 0).

The syntax that allows us to create strings with double quotes (") is just a helper (syntactic sugar); we can create strings just as any other array, and we can work with them the same. Let's see an example:

#include <stdio.h>

int main(void)
{
  char alphabet[27]; // 26 places for letters + 1 for temrinating 0

  for (int i = 0; i < 26; ++i)
    alphabet[i] = 'A' + i;

  alphabet[26] = 0; // terminate the string

  puts(alphabet);

  return 0;
}

alphabet is an array of chars, i.e. a string. Its length is 27 because we need 26 places for letters and one extra space for the terminating 0. Here it's important to remind ourselves that we count from 0, so the alphabet can be indexed from 0 to 26, i.e. 26 is the last index we can use, doing alphabet[27] would be an error! Next we fill the array with letters (see how we can treat chars as numbers and do 'A' + i). We iterate while i < 26, i.e. we will fill all the places in the array up to the index 25 (including) and leave the last place (with index 26) empty for the terminating 0. That we subsequently assign. And finally we print the string with puts(alphabet) -- here note that there are no double quotes around alphabet because its a variable name. Doing puts("alphabet") would cause the program to literally print out alphabet. Now the program outputs:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

In C there is a standard library for working with strings called string (#include <string.h>), it contains such function as strlen for computing string length or strcmp for comparing strings.

One final example -- a creature generator -- will show all the three new data types in action:

#include <stdio.h>
#include <stdlib.h> // for rand()

typedef struct
{
  char name[4]; // 3 letter name + 1 place for 0
  int weightKg;
  int legCount;
} Creature; // some weird creature

Creature creatures[100]; // global array of Creatures

void printCreature(Creature c)
{
  printf("Creature named %s ",c.name); // %s prints a string
  printf("(%d kg, ",c.weightKg);
  printf("%d legs)\n",c.legCount);
}

int main(void)
{
  // generate random creatures:

  for (int i = 0; i < 100; ++i)
  {
    Creature c;

    c.name[0] = 'A' + (rand() % 26);
    c.name[1] = 'a' + (rand() % 26);
    c.name[2] = 'a' + (rand() % 26);
    c.name[3] = 0; // terminate the string

    c.weightKg = 1 + (rand() % 1000); 
    c.legCount = 1 + (rand() % 10); // 1 to 10 legs

    creatures[i] = c;
  }

  // print the creatures:

  for (int i = 0; i < 100; ++i)
    printCreature(creatures[i]);

  return 0;
}

When run you will see a list of 100 randomly generated creatures which may start e.g. as:

Creature named Nwl (916 kg, 4 legs)
Creature named Bmq (650 kg, 2 legs)
Creature named Cda (60 kg, 4 legs)
Creature named Owk (173 kg, 7 legs)
Creature named Hid (430 kg, 3 legs)
...

Macros/Preprocessor

The C language comes with a feature called preprocessor which is necessary for some advanced things. It allows automatized modification of the source code before it is compiled.

Remember how we said that compiler compiles C programs in several steps such as generating object files and linking? There is one more step we didn't mention: preprocessing. It is the very first step -- the source code you give to the compiler first goes to the preprocessor which modifies it according to special commands in the source code called preprocessor directives. The result of preprocessing is a pure C code without any more preprocessing directives, and this is handed over to the actual compilation.

The preprocessor is like a mini language on top of the C language, it has its own commands and rules, but it's much more simple than C itself, for example it has no data types or loops.

Each directive begins with #, is followed by the directive name and continues until the end of the line (\ can be used to extend the directive to the next line).

We have already encountered one preprocessor directive: the #include directive when we included library header files. This directive pastes a text of the file whose name it is handed to the place of the directive.

Another directive is #define which creates so called macro -- in its basic form a macro is nothing else than an alias, a nickname for some text. This is used to create constants. Consider the following code:

#include <stdio.h>

#define ARRAY_SIZE 10

int array[ARRAY_SIZE];

void fillArray(void)
{
  for (int i = 0; i < ARRAY_SIZE; ++i)
    array[i] = i;
}

void printArray(void)
{
  for (int i = 0; i < ARRAY_SIZE; ++i)
    printf("%d ",array[i]);
}

int main()
{
  fillArray();
  printArray();
  return 0;
}

#define ARRAY_SIZE 10 creates a macro that can be seen as a constant named ARRAY_SIZE which stands for 10. From this line on any occurrence of ARRAY_SIZE that the preprocessor encounters in the code will be replaced with 10. The reason for doing this is obvious -- we respect the DRY (don't repeat yourself) principle, if we didn't use a constant for the array size and used the direct numeric value 10 in different parts of the code, it would be difficult to change them all later, especially in a very long code, there's a danger we'd miss some. With a constant it is enough to change one line in the code (e.g. #define ARRAY_SIZE 10 to #define ARRAY_SIZE 20).

The macro substitution is literally a glorified copy-paste text replacement, there is nothing very complex going on. This means you can create a nickname for almost anything (for example you could do #define when if and then also use when in place of if -- but it's probably not a very good idea). By convention macro names are to be ALL_UPPER_CASE (so that whenever you see an all upper case word in the source code, you know it's a macro).

Macros can optionally take parameters similarly to functions. There are no data types, just parameter names. The usage is demonstrated by the following code:

#include <stdio.h>

#define MEAN3(a,b,c) (((a) + (b) + (c)) / 3) 

int main()
{
  int n = MEAN3(10,20,25);

  printf("%d\n",n);

  return 0;
}

MEAN3 computes the mean of 3 values. Again, it's just text replacement, so the line int n = MEAN3(10,20,25); becomes int n = (((10) + (20) + (25)) / 3); before code compilation. Why are there so many brackets in the macro? It's always good to put brackets over a macro and all its parameters because the parameters are again a simple text replacement; consider e.g. a macro #define HALF(x) x / 2 -- if it was invoked as HALF(5 + 1), the substitution would result in the final text 5 + 1 / 2, which gives 5 (instead of the intended value 3).

You may be asking why would we use a macro when we can use a function for computing the mean? Firstly macros don't just have to work with numbers, they can be used to generate parts of the source code in ways that functions can't. Secondly using a macro may sometimes be simpler, it's shorter and will be faster to execute because the is no function call (which has a slight overhead) and because the macro expansion may lead to the compiler precomputing expressions at compile time. But beware: macros are usually worse than functions and should only be used in very justified cases. For example macros don't know about data types and cannot check them, and they also result in a bigger compiled executable (function code is in the executable only once whereas the macro is expanded in each place where it is used and so the code it generates multiplies).

Another very useful directive is #if for conditional inclusion or exclusion of parts of the source code. It is similar to the C if command. The following example shows its use:

#include <stdio.h>

#define RUDE 0

void printNumber(int x)
{
  puts(
#if RUDE
    "You idiot, the number is:"
#else
    "The number is:"
#endif
  );

  printf("%d\n",x);
}

int main()
{
  printNumber(3);
  printNumber(100);

#if RUDE
  puts("Bye bitch.");
#endif

  return 0;
}

When run, we get the output:

The number is:
3
The number is:
100

And if we change #define RUDE 0 to #define RUDE 1, we get:

You idiot, the number is:
3
You idiot, the number is:
100
Bye bitch.

We see the #if directive has to have a corresponding #endif directive that terminates it, and there can be an optional #else directive for an else branch. The condition after #if can use similar operators as those in C itself (+, ==, &&, || etc.). There also exists an #ifdef directive which is used the same and checks if a macro of given name has been defined.

#if directives are very useful for conditional compilation, they allow for creation of various "settings" and parameters that can fine-tune a program -- you may turn specific features on and off with this directive. It is also helpful for portability; compilers may automatically define specific macros depending on the platform (e.g. _WIN64, __APPLE__, ...) based on which you can trigger different code. E.g.:

#ifdef _WIN64
  puts("Your OS sucks.");
#endif

Let us talk about one more thing that doesn't fall under the preprocessor language but is related to constants: enumerations. Enumeration is a data type that can have values that we specify individually, for example:

typedef enum
{
  APPLE,
  PEAR,
  TOMATO
} Fruit;

This creates a new data type Fruit. Variables of this type may have values APPLE, PEAR or TOMATO, so we may for example do Fruit myFruit = APPLE;. These values are in fact integers and the names we give them are just nicknames, so here APPLE is equal to 0, PEAR to 1 and TOMATO to 2.

Pointers

Pointers are an advanced topic that many people fear -- many complain they're hard to learn, others complain about memory unsafety and potential dangers of using pointers. These people are stupid, pointers are great.

But beware, there may be too much new information in the first read. Don't get scared, give it some time.

Pointers allow us to do certain advanced things such as allocate dynamic memory, return multiple values from functions, inspect content of memory or use functions in similar ways in which we use variables.

A pointer is essentially nothing complicated: it is a data type that can hold a memory address (plus an information about what data type should be stored at that address). An address is simply a number. Why can't we just use an int to store a memory address? Because the size of int and a pointer may differ, the size of pointer depends on each platform's address width. Besides this, as said, a pointer actually holds not only an address but also the information about the type it points to, which is a safety mechanism that will become clear later. It is also good when the compiler knows a certain variable is supposed to point to a memory rather than to hold a generic number -- this can all prevent bugs. I.e. pointers and generic integers are distinguished for the same reason other data types are distinguished -- in theory they don't have to be distinguished, but it's safer.

It is important to stress again that a pointer is not a pure address but it also knows about the data type it is pointing to, so there are many kinds of pointers: a pointer to int, a pointer to char, a pointer to a specific struct type etc.

A variable of pointer type is created similarly to a normal variable, we just add * after the data type, for example int *x; creates a variable named x that is a pointer to int (some people would write this as int* x;).

But how do we assign a value to the pointer? To do this, we need an address of something, e.g. of some variable. To get an address of a variable we use the & character, i.e. &a is the address of a variable a.

The last basic thing we need to know is how to dereference a pointer. Dereferencing means accessing the value at the address that's stored in the pointer, i.e. working with the pointed to value. This is again done (maybe a bit confusingly) with * character in front of a pointer, e.g. if x is a pointer to int, *x is the int value to which the pointer is pointing. An example can perhaps make it clearer.

#include <stdio.h>

int main(void)
{
  int normalVariable = 10;
  int *pointer;

  pointer = &normalVariable;

  printf("address in pointer: %p\n",pointer);
  printf("value at this address: %d\n",*pointer);

  *pointer = *pointer + 10;

  printf("normalVariable: %d\n",normalVariable);

  return 0;
}

This may print e.g.:

address in pointer: 0x7fff226fe2ec
value at this address: 10
normalVariable: 20

int *pointer; creates a pointer to int with name pointer. Next we make the pointer point to the variable normalVariable, i.e. we get the address of the variable with &normalVariable and assign it normally to pointer. Next we print firstly the address in the pointer (accessed with pointer) and the value at this address, for which we use dereference as *pointer. At the next line we see that we can also use dereference for writing to the pointed address, i.e. doing *pointer = *pointer + 10; here is the same as doing normalVariable = normalVariable + 10;. The last line shows that the value in normalVariable has indeed changed.

IMPORTANT NOTE: You generally cannot read and write from/to random addresses! This will crash your program. To be able to write to a certain address it must be allocated, i.e. reserved for use. Addresses of variables are allocated by the compiler and can be safely operated with.

There's a special value called NULL (a macro defined in the standard library) that is meant to be assigned to pointer that points to "nothing". So when we have a pointer p that's currently not supposed to point to anything, we do p = NULL;. In a safe code we should always check (with if) whether a pointer is not NULL before dereferencing it, and if it is, then NOT dereference it. This isn't required but is considered a "good practice" in safe code, storing NULL in pointers that point nowhere prevents dereferencing random or unallocated addresses which would crash the program.

But what can pointers be good for? Many things, for example we can kind of "store variables in variables", i.e. a pointer is a variable which says which variable we are now using, and we can switch between variable any time. E.g.:

#include <stdio.h>

int bankAccountMonica = 1000;
int bankAccountBob = -550;
int bankAccountJose = 700;

int *payingAccount; // pointer to who's currently paying

void payBills(void)
{
  *payingAccount -= 200;
}

void buyFood(void)
{
  *payingAccount -= 50;
}

void buyGas(void)
{
  *payingAccount -= 20;
}

int main(void)
{
  // let Jose pay first

  payingAccount = &bankAccountJose;

  payBills();
  buyFood();
  buyGas();

  // that's enough, now let Monica pay 

  payingAccount = &bankAccountMonica;

  buyFood();
  buyGas();
  buyFood();
  buyFood();

  // now it's Bob's turn

  payingAccount = &bankAccountBob;

  payBills();
  buyFood();
  buyFood();
  buyGas();

  printf("Monika has $%d left.\n",bankAccountMonica);
  printf("Jose has $%d left.\n",bankAccountJose);
  printf("Bob has $%d left.\n",bankAccountBob);

  return 0;
}

Well, this could be similarly achieved with arrays, but pointers have more uses. For example they allow us to return multiple values by a function. Again, remember that we said that (with the exception of arrays) a function cannot modify a variable passed to it because it always makes its own local copy of it? We can bypass this by, instead of giving the function the value of the variable, giving it the address of the variable. The function can read the value of that variable (with dereference) but it can also CHANGE the value, it simply writes a new value to that address (again, using dereference). This example shows it:

#include <stdio.h>
#include <math.h>

#define PI 3.141592

// returns 2D coordinates of a point on a unit circle
void getUnitCirclePoint(float angle, float *x, float *y)
{
  *x = sin(angle);
  *y = cos(angle);
}

int main(void)
{
  for (int i = 0; i < 8; ++i)
  {
    float pointX, pointY;

    getUnitCirclePoint(i * 0.125 * 2 * PI,&pointX,&pointY);

    printf("%lf %lf\n",pointX,pointY);
  }

  return 0;
}

Function getUnitCirclePoint doesn't return any value in the strict sense, but thank to pointers it effectively returns two float values via its parameters x and y. These parameters are of the data type pointer to float (as there's * in front of them). When we call the function with getUnitCirclePoint(i * 0.125 * 2 * PI,&pointX,&pointY);, we hand over the addresses of the variables pointX and pointY (which belong to the main function and couldn't normally be accessed in getUnitCirclePoint). The function can then compute values and write them to these addresses (with dereference, *x and *y), changing the values in pointX and pointY, effectively returning two values.

Now let's take a look at pointers to structs. Everything basically works the same here, but there's one thing to know about, a syntactic sugar known as an arrow (->). Example:

#include <stdio.h>

typedef struct
{
  int a;
  int b;
} SomeStruct;

SomeStruct s;
SomeStruct *sPointer;

int main(void)
{
  sPointer = &s;

  (*sPointer).a = 10; // without arrow
  sPointer->b = 20;   // same as (*sPointer).b = 20

  printf("%d\n",s.a);
  printf("%d\n",s.b);

  return 0;
}

Here we are trying to write values to a struct through pointers. Without using the arrow we can simply dereference the pointer with *, put brackets around and access the member of the struct normally. This shows the line (*sPointer).a = 10;. Using an arrow achieves the same thing but is perhaps a bit more readable, as seen in the line sPointer->b = 20;. The arrow is simply a special shorthand and doesn't need any brackets.

Now let's talk about arrays -- these are a bit special. The important thing is that an array is itself basically a pointer. What does this mean? If we create an array, let's say int myArray[10];, then myArray is basically a pointer to int in which the address of the first array item is stored. When we index the array, e.g. like myArray[3] = 1;, behind the scenes there is basically a dereference because the index 3 means: 3 places after the address pointed to by myArray. So when we index an array, the compiler takes the address stored in myArray (the address of the array start) and adds 3 to it (well, kind of) by which it gets the address of the item we want to access, and then dereferences this address.

Arrays and pointer are kind of a duality -- we can also use array indexing with pointers. For example if we have a pointer declared as int *x;, we can access the value x points to with a dereference (*x), but ALSO with indexing like this: x[0]. Accessing index 0 simply means: take the value stored in the variable and add 0 to it, then dereference it. So it achieves the same thing. We can also use higher indices (e.g. x[10]), BUT ONLY if x actually points to a memory that has at least 11 allocated places.

This leads to a concept called pointer arithmetic. Pointer arithmetic simply means we can add or subtract numbers to pointer values. If we continue with the same pointer as above (int *x;), we can actually add numbers to it like *(x + 1) = 10;. What does this mean?! It means exactly the same thing as x[1]. Adding a number to a pointer shifts that pointer given number of places forward. We use the word places because each data type takes a different space in memory, for example char takes one byte of memory while int takes usually 4 (but not always), so shifting a pointer by N places means adding N times the size of the pointed to data type to the address stored in the pointer.

This may be a lot information to digest. Let's provide an example to show all this in practice:

#include <stdio.h>

// our own string print function
void printString(char *s)
{
  int position = 0;

  while (s[position] != 0)
  {
    putchar(s[position]);
    position += 1;
  }
}

// returns the length of string s
int stringLength(char *s)
{
  int length = 0;

  while (*s != 0) // count until terminating 0
  {
    length += 1;
    s += 1; // shift the pointer one character to right
  }

  return length;
}

int main(void)
{
  char testString[] = "catdog";

  printString("The string '");
  printString(testString);
  printString("' has length ");

  int l = stringLength(testString);

  printf("%d.",l);

  return 0;
}

The output is:

The string 'catdog' has length 6.

We've created a function for printing strings (printString) similar to puts and a function for computing the length of a string (stringLength). They both take as an argument a pointer to char, i.e. a string. In printString we use indexing ([ and ]) just as if s was an array, and indeed we see it works! In stringLength we similarly iterate over all characters in the string but we use dereference (*s) and pointer arithmetic (s += 1;). It doesn't matter which of the two styles we choose -- here we've shown both, for educational purposes. Finally notice that the string we actually work with is created in main as an array with char testString[] = "catdog"; -- here we don't need to specify the array size between [ and ] because we immediately assign a string literal to it ("catdog") and in such a case the compiler knows how big the array needs to be and automatically fills in the correct size.

Now that know about pointers, we can finally completely explain the functions from stdio we've been using:

int puts(char *s): A simple and fast function for printing a string (adds the newline character \n at the end).
int printf(char *format, ...): A little bit more complex function that can print not only strings but also other data types. It takes a variable number of parameters. The first one is always a string that specifies the print format -- this string can contain special sequences that will be replaced by textual representations of values we additionally provide as extra parameters after format. E.g. the sequence "%d" is replaced with a number obtained from the value of a corresponding int parameter. Similarly %c is for char, %s for strings, %p for pointers. Example: printf("MyInt = %d, myChar = %c, MyStr = %s\n",myInt,myChar,myStr);.
int getchar(void): Reads a single text character from the input and returns it. Why does the function return int and not char? Because the function can return additional special values such as EOF (end of file) which couldn't be stored in plain char.
int scanf(char *format, ...): Function for reading various data types from the input. Like printf it takes a variable number of parameters. The first one is a string that specifies which data type(s) to read -- this is a bit complicated but "%d" reads an int, "%f" float, "%c" char and "%s" string. The following arguments are pointers to expected data types, so e.g. if we've provided the format string "%d", a pointer to int has to follow. Through this parameter the value that's been read will be returned (in the same way we've seen in one example above).

Nigger.

Files

Now we'll take a look at how we can read and write from/to files on the computer disk which enables us to store information permanently or potentially process data such as images or audio. Files aren't so difficult.

We work with files through functions provided in the stdio library (so it has to be included). We distinguish two types of files:

text files: Contain text, are human readable.
binary files: Contain binary data, aren't human readable, are more efficient but also more prone to corruption.

From programmer's point of view there's actually not a huge difference between the two, they're both just sequences of characters or bytes (which are kind of almost the same). Text files are a little more abstract, they handle potentially different format of newlines etc. The main thing for us is that we'll use slightly different functions for each type.

There is a special data type for file called FILE (we'll be using a pointer to it). Whatever file we work with, we need to firstly open it with the function fopen and when we're done with it, we need to close it with a function fclose.

First we'll write something to a text file:

#include <stdio.h>

int main(void)
{
  FILE *textFile = fopen("test.txt","w"); // "w" for write

  if (textFile != NULL) // if opened successfully
    fprintf(textFile,"Hello file.");
  else
    puts("ERROR: Couldn't open file.");

  fclose(textFile);

  return 0;
}

When run, the program should create a new file named test.txt in the same directory we're in and in it you should find the text Hello file.. FILE *textFile creates a new variable textFile which is a pointer to the FILE data type. We are using a pointer simply because the standard library is designed this way, its functions work with pointers (it can be more efficient). fopen("test.txt","w"); attempts to open the file test.txt in text mode for writing -- it returns a pointer that represents the opened file. The mode, i.e. text/binary, read/write etc., is specified by the second argument: "w"; w simply specifies write and the text mode is implicit (it doesn't have to be specified). if (textFile != NULL) checks if the file has been successfully opened; the function fopen returns NULL (the value of "point to nothing" pointers) if there was an error with opening the file (such as that the file doesn't exist). On success we write text to the file with a function fprintf -- it's basically the same as printf but works on files, so it's first parameter is always a pointer to a file to which it should write. You can of course also print numbers and anything that printf can with this function. Finally we mustn't forget to close the file at the end with fclose!

Now let's write another program that reads the file we've just created and writes its content out in the command line:

#include <stdio.h>

int main(void)
{
  FILE *textFile = fopen("test.txt","r"); // "r" for read

  if (textFile != NULL) // if opened successfully
  {
    char c;

    while (fscanf(textFile,"%c",&c) != EOF) // while not end of file
      putchar(c);
  }
  else
    puts("ERROR: Couldn't open file.");

  fclose(textFile);

  return 0;
}

Notice that in fopen we now specify "r" (read) as a mode. Again, we check if the file has been opened successfully (if (textFile != NULL)). If so, we use a while loop to read and print all characters from the file until we encounter the end of file. The reading of file characters is done with the fscanf function inside the loop's condition -- there's nothing preventing us from doing this. fscanf again works the same as scanf (so it can read other types than only chars), just on files (its first argument is the file to read from). On encountering end of file fscanf returns a special value EOF (which is macro constant defined in the standard library). Again, we must close the file at the end with fclose.

We will now write to a binary file:

#include <stdio.h>

int main(void)
{
  unsigned char image[] = // image in ppm format
  { 
    80, 54, 32, 53, 32, 53, 32, 50, 53, 53, 32,
    255,255,255, 255,255,255, 255,255,255, 255,255,255, 255,255,255,
    255,255,255,    0, 0,  0, 255,255,255,   0,  0,  0, 255,255,255,
    255,255,255, 255,255,255, 255,255,255, 255,255,255, 255,255,255,
      0,  0,  0, 255,255,255, 255,255,255, 255,255,255,   0,  0,  0,
    255,255,255,   0,  0,  0,   0,  0,  0,   0,  0,  0, 255,255,255
  };

  FILE *binFile = fopen("image.ppm","wb");

  if (binFile != NULL) // if opened successfully
    fwrite(image,1,sizeof(image),binFile);
  else
    puts("ERROR: Couldn't open file.");

  fclose(binFile);

  return 0;
}

Okay, don't get scared, this example looks complex because it is trying to do a cool thing: it creates an image file! When run, it should produce a file named image.ppm which is a tiny 5x5 smiley face image in ppm format. You should be able to open the image in any good viewer (I wouldn't bet on Windows programs though). The image data was made manually and are stored in the image array. We don't need to understand the data, we just know we have some data we want to write to a file. Notice how we can manually initialize the array with values using { and } brackets. We open the file for writing and in binary mode, i.e. with a mode "wb", we check the success of the action and then write the whole array into the file with one function call. The function is name fwrite and is used for writing to binary files (as opposed to fprintf for text files). fwrite takes these parameters: pointer to the data to be written to the file, size of one data element (in bytes), number of data elements and a pointer to the file to write to. Our data is the image array and since "arrays are basically pointers", we provide it as the first argument. Next argument is 1 (unsigned char always takes 1 byte), then length of our array (sizeof is a special operator that substitutes the size of a variable in bytes -- since each item in our array takes 1 byte, sizeof(image) provides the number of items in the array), and the file pointer. At the end we close the file.

And finally we'll finish with reading this binary file back:

#include <stdio.h>

int main(void)
{
  FILE *binFile = fopen("image.ppm","rb");

  if (binFile != NULL) // if opened successfully
  {
    unsigned char byte;

    while (fread(&byte,1,1,binFile))
      printf("%d ",byte);

    putchar('\n');
  }
  else
    puts("ERROR: Couldn't open file.");

  fclose(binFile);

  return 0;
}

The file mode is now "rb" (read binary). For reading from binary files we use the fread function, similarly to how we used fscanf for reading from a text file. fread has these parameters: pointer where to store the read data (the memory must have sufficient space allocated!), size of one data item, number of items to read and the pointer to the file which to read from. As the first argument we pass &byte, i.e. the address of the variable byte, next 1 (we want to read a single byte whose size in bytes is 1), 1 (we want to read one byte) and the file pointer. fread returns the number of items read, so the while condition holds as long as fread reads bytes; once we reach end of file, fread can no longer read anything and returns 0 (which in C is interpreted as a false value) and the loop ends. Again, we must close the file at the end.

More On Functions (Recursion, Function Pointers)

There's more to be known about functions.

An important concept in programming is recursion -- the situation in which a function calls itself. Yes, it is possible, but some rules have to be followed.

When a function calls itself, we have to ensure that we won't end up in infinite recursion (i.e. the function calls itself which subsequently calls itself and so on until infinity). This crashes our program. There always has to be a terminating condition in a recursive function, i.e. an if branch that will eventually stop the function from calling itself again.

But what is this even good for? Recursion is actually very common in math and programming, many problems are recursive in nature. Many things are beautifully described with recursion (e.g. fractals). But remember: anything a recursion can achieve can also be achieved by iteration (loop) and vice versa. It's just that sometimes one is more elegant or more computationally efficient.

Let's see this on a typical example of the mathematical function called factorial. Factorial of N is defined as N x (N - 1) x (N - 2) x ... x 1. It can also be defined recursively as: factorial of N is 1 if N is 0, otherwise N times factorial of N - 1. Here is some code:

#include <stdio.h>

unsigned int factorialRecursive(unsigned int x)
{
  if (x == 0) // terminating condition
    return 1;
  else
    return x * factorialRecursive(x - 1);
}

unsigned int factorialIterative(unsigned int x)
{
  unsigned int result = 1;

  while (x > 1)
  {
    result *= x;
    x--;
  }

  return result;
}

int main(void)
{
  printf("%d %d\n",factorialRecursive(5),factorialIterative(5));
  return 0;
}

factorialIterative computes the factorial by iteration. factorialRecursive uses recursion -- it calls itself. The important thing is the recursion is guaranteed to end because every time the function calls itself, it passes a decremented argument so at one point the function will receive 0 in which case the terminating condition (if (x == 0)) will be triggered which will avoid the further recursive call.

It should be mentioned that performance-wise recursion is almost always worse than iteration (function calls have certain overhead), so in practice it is used sparingly. But in some cases it is very well justified (e.g. when it makes code much simpler while creating unnoticeable performance loss).

Another thing to mention is that we can have pointers to functions; this is an advanced topic so we'll stay at it just briefly. Function pointers are pretty powerful, they allow us to create so called callbacks: imagine we are using some GUI framework and we want to tell it what should happen when a user clicks on a specific button -- this is usually done by giving the framework a pointer to our custom function that it will be called by the framework whenever the button is clicked.

Dynamic Allocation (Malloc)

Dynamic memory allocation means the possibility of reserving additional memory (RAM) for our program at run time, whenever we need it. This is opposed to static memory allocation, i.e. reserving memory for use at compile time (when compiling, before the program runs). We've already been doing static allocation whenever we created a variable -- compiler automatically reserves as much memory for our variables as is needed. But what if we're writing a program but don't yet know how much memory it will need? Maybe the program will be reading a file but we don't know how big that file is going to be -- how much memory should we reserve? Dynamic allocation allows us to reserve this memory with functions when the program is actually runing and already knows how much of it should be reserved.

It must be known that dynamic allocation comes with a new kind of bug known as a memory leak. It happens when we reserve a memory and forget to free it after we no longer need it. If this happens e.g. in a loop, the program will continue to "grow", eat more and more RAM until operating system has no more to give. For this reason, as well as others such as simplicity, it may sometimes be better to go with only static allocation.

Anyway, let's see how we can allocate memory if we need to. We use mostly just two functions that are provided by the stdlib library. One is malloc which takes as an argument size of the memory we want to allocate (reserve) in bytes and returns a pointer to this allocated memory if successful or NULL if the memory couldn't be allocated (which in serious programs we should always check). The other function is free which frees the memory when we no longer need it (every allocated memory should be freed at some point) -- it takes as the only parameter a pointer to the memory we've previously allocated. There is also another function called realloc which serves to change the size of an already allocated memory: it takes a pointer the the allocated memory and the new size in byte, and returns the pointer to the resized memory.

Here is an example:

#include <stdio.h>
#include <stdlib.h>

#define ALLOCATION_CHUNK 32 // by how many bytes to resize

int main(void)
{
  int charsRead = 0;
  int resized = 0; // how many times we called realloc
  char *inputChars = malloc(ALLOCATION_CHUNK * sizeof(char)); // first allocation

  while (1) // read input characters
  {
    char c = getchar();
 
    charsRead++;

    if ((charsRead % ALLOCATION_CHUNK) == 0)
    {
      inputChars = // we need more space, resize the array
        realloc(inputChars,(charsRead / ALLOCATION_CHUNK + 1) * ALLOCATION_CHUNK * sizeof(char));

      resized++;
    }

    inputChars[charsRead - 1] = c;

    if (c == '\n')
    {
      charsRead--; // don't count the last newline character
      break;
    }
  }

  puts("The string you entered backwards:");

  while (charsRead > 0)
  {
    putchar(inputChars[charsRead - 1]);
    charsRead--;
  }

  free(inputChars); // important!

  putchar('\n');
  printf("I had to resize the input buffer %d times.",resized);

  return 0;
}

This code reads characters from the input and stores them in an array (inputChars) -- the array is dynamically resized if more characters are needed. (We restrain from calling the array inputChars a string because we never terminate it with 0, we couldn't print it with standard functions like puts.) At the end the entered characters are printed backwards (to prove we really stored all of them), and we print out how many times we needed to resize the array.

We define a constant (macro) ALLOCATION_CHUNK that says by how many characters we'll be resizing our character buffer. I.e. at the beginning we create a character buffer of size ALLOCATION_CHUNK and start reading input character into it. Once it fills up we resize the buffer by another ALLOCATION_CHUNK characters and so on. We could be resizing the buffer by single characters but that's usually inefficient (the function malloc may be quite complex and take some time to execute).

The line starting with char *inputChars = malloc(... creates a pointer to char -- our character buffer -- to which we assign a chunk of memory allocated with malloc. Its size is ALLOCATION_CHUNK * sizeof(char). Note that for simplicity we don't check if inputChars is not NULL, i.e. whether the allocation succeeded -- but in your program you should do it :) Then we enter the character reading loop inside which we check if the buffer has filled up (if ((charsRead % ALLOCATION_CHUNK) == 0)). If so, we used the realloc function to increase the size of the character buffer. The important thing is that once we exit the loop and print the characters stored in the buffer, we free the memory with free(inputChars); as we no longer need it.

Debugging, Optimization

Debugging means localizing and fixing bugs (errors) in your program. In practice there are always bugs, even in very short programs (you've probably already figured that out yourself), some small and insignificant and some pretty bad ones that make your program unusable, vulnerable or even dangerous.

There are two kinds of bugs: syntactic errors and semantic errors. A syntactic error is when you write something not obeying the C grammar, it's like a typo or grammatical error in a normal language -- these errors are very easy to detect and fix, a compiler won't be able to understand your program and will point you to the exact place where the error occurs. A semantic error can be much worse -- it's a logical error in the program; the program will compile and run but the program will behave differently than intended. The program may crash, leak memory, give wrong results, run slowly, corrupt files etc. These errors may be hard to spot and fix, especially when they happen in rare situations. We'll be only considering semantic errors from now on.

If we spot a bug, how do we fix it? The first thing is to find a way to replicate it, i.e. find the exact steps we need to make with the program to make the bug appear (e.g. "in the menu press keys A and B simultaneously", ...). Next we need to trace and locate which exact line or piece of code is causing the bug. This can either be done with the help of specialized debuggers such as gdb or valgrind, but there's usually a much easier way of using printing functions such as printf. (Still do check out the above mentioned debuggers, they're very helpful.)

Let's say your program crashes and you don't know at which line. You simply put prints such as printf("A\n"); and printf("B\n); at the beginning and end of a code you suspect might be causing the crash. Then you run the program: if A is printed but B isn't, you know the crash happened somewhere between the two prints, so you shift the B print a little bit up and so on until you find exactly after which line B stops printing -- this is the line that crashes the program. IMPORTANT NOTE: the prints have to have newline (\n) at the end, otherwise this method may not work because of output buffering.

Of course, you may use the prints in other ways, for example to detect at which place a value of variable changes to a wrong value. (Asserts are also good for keeping an eye on correct values of variables.)

What if the program isn't exactly crashing but is giving wrong results? Then you need to trace the program step by step (not exactly line by line, but maybe function by function) and check which step has a problem in it. If for example your game AI is behaving stupid, you firstly check (with prints) if it correctly detects its circumstances, then you check whether it makes the correct decision based on the circumstances, then you check whether the pathfinding algorithm finds the correct path etc. At each step you need to know what the correct behavior should be and you try to find where the behavior is broken.

Knowing how to fix a bug isn't everything, we also need to find the bugs in the first place. Testing is the process of trying to find bugs by simply running and using the program. Remember, testing can't prove there are no bugs in the program, it can only prove bugs exist. You can do testing manually or automate the tests. Automated tests are very important for preventing so called regressions (so the tests are called regression tests). Regression happens when during further development you break some of its already working features (it is very common, don't think it won't be happening to you). Regression test (which can simply be just a normal C program) simply automatically checks whether the already implemented functions still give the same results as before (e.g. if sin(0) = 0 etc.). These tests should be run and pass before releasing any new version of the program (or even before any commit of new code).

Optimization is also a process of improving an already working program, but here we try to make the program more efficient -- the most common goal is to make the program faster, smaller or consume less RAM. This can be a very complex task, so we'll only mention it briefly.

The very basic thing we can do is to turn on automatic optimization with a compiler flag: -O3 for speed, -Os for program size (-O2 and -O1 are less aggressive speed optimizations). Yes, it's that simple, you simply add -O3 and your program gets magically faster. Remember that optimizations against different resources are often antagonistic, i.e. speeding up your program typically makes it consume more memory and vice versa. You need to choose. Optimizing manually is a great art. Let's suppose you are optimizing for speed -- the first, most important thing is to locate the part of code that's slowing down you program the most, so called bottleneck. That is the code you want to make faster. Trying to optimize non-bottlenecks doesn't speed up your program as a whole much; imagine you optimize a part of the code that takes 1% of total execution time by 200% -- your program will only get 0.5% faster. Bottlenecks can be found using profiling -- measuring the execution time of different parts of the program (e.g. each function). This can be done manually or with tools such a gprof. Once you know where to optimize, you try to apply different techniques: using algorithms with better time complexity, using look up tables, optimizing cache behavior and so on. This is beyond the scope of this tutorial.

Final Program

Now is the time to write a final program that showcases what we've learned, so let's write a quite simple but possibly useful hex viewer. The program will allow us to interactively inspect bytes in any file, drawing their hexadecimal values along with their addresses, supporting one character commands to move within the file. If you want, you can take it and improve it as an exercise, for example by adding more viewing modes (showing decimal octal or ASCII values), maybe even allowing editing and saving the file. Here is the source code:

/* Simple interactive hex file viewer. */

#include <stdio.h>
#include <stdlib.h>

#define ROWS 10 // how many rows and columns to print at one screen
#define COLS 16

unsigned char *fileContent = NULL;   // this will contain the loaded file
unsigned long long fileSize = 0;     // size of fileContent in bytes
unsigned long long fileOffset = 0;   // current offset within the file
const char *fileName = NULL;

// Loads file with given name into fileContent, returns 1 on success, 0 on fail.
int loadFile(const char *fileToOpen)
{
  FILE *file = fopen(fileToOpen,"rb");

  if (file == NULL)
    return 0;

  fseek(file,0L,SEEK_END); // get to the end of the file
  fileSize = ftell(file);  // our position now says the size of the file
  rewind(file);            // get back to start of the file

  fileContent = malloc(fileSize);  // allocate memory to load the file into

  if (fileContent == NULL)
  {
    fclose(file); // don't forget to close the file
    return 0;
  }

  if (fread(fileContent,1,fileSize,file) != fileSize)
  {
    fclose(file);
    free(fileContent);
    return 0;
  }

  fclose(file);

  return 1;
}

// Call when loaded file is no longer needed.
void unloadFile(void)
{
  free(fileContent); // free the allocated memory
}

// Draws the current screen, i.e. hex view of the file at current offset.
void drawScreen(void)
{
  for (int i = 0; i < 80; ++i) // scroll the old screen our of the view
    putchar('\n');

  printf("%s: %llu / %llu\n\n",fileName,fileOffset,fileSize);

  unsigned long long offset = fileOffset;

  for (int i = 0; i < ROWS * COLS; ++i)
  {
    if (offset % COLS == 0)
      printf("%04X    ",(int) offset);

    if (offset < fileSize)
      printf("%02X ",fileContent[offset]);
    else
      printf(".. ");

    offset++;

    if (offset % COLS == 0) // break line after each COLS values
      putchar('\n');
  }
}

int main(int argc, char **argv)
{
  if (argc < 2)
  {
    puts("ERROR: please pass a file to open");
    return 1;
  }

  fileName = argv[1]; // (argv[0] is the name of our program, we want argv[1])

  if (!loadFile(fileName))
  {
    printf("ERROR: couldn't open the file \"%s\"\n",fileName);
    return 1;
  }

  int goOn = 1;

  while (goOn) // the main interactive loop
  {
    drawScreen();

    puts("\ntype command (w = end, s = start, a = back, d = next, q = quit)");

    char userInput = getchar();

    switch (userInput)
    {
      case 'q':
        goOn = 0;
        break;

      case 's':
        if (fileOffset + COLS < fileSize)
          fileOffset += COLS;

          break;

      case 'w': 
        if (fileOffset >= COLS)
          fileOffset -= COLS;

        break;

      case 'a': 
        fileOffset = 0;
        break;

      case 'd':
        fileOffset = ((fileSize - COLS) / COLS) * COLS; // aligns the offset
        break;

      default: 
        puts("unknown command, sorry");
        break;
    }
  }

  unloadFile();

  return 0;
}

To add a few comments: the program opens a file whose name it gets passed as a command line argument, so it is used as: ./hexview myfile. We try to correctly perform all safety checks, e.g. if we actually get passed the file name, if we manage to open it and so on. Our program (a bit inefficiently) loads the whole file into memory (advanced programs only load parts of the file) -- for this it first checks the file size, allocates sufficient memory for it with malloc (also checking for errors) and loads it there. Then we have a function to draw the current file view and inside the main program body we have an interactive loop that loads and handles user commands and issues the view drawing. That is basically it!

Bonus: Introduction To Graphics (ASCII, PPM, SDL2, ...)

Let's stress you should only get into graphics after you've written several purely command-line programs and are comfortable with the language. Don't try graphics programming until you can easily work with 2D arrays, structs and so on. Graphics is a huge topic of its own, there is so much we can't cover here, remember this is just a quick, basic starting point for making pictures with C.

For start please note that:

C itself doesn't know anything about graphics. C is just trying to be a good programming language, it leaves the vast area of graphics for others to solve, therefore though you can try to avoid it (see below), typically you will use a third party library to draw some real pixels to the screen, there isn't a universal way of doing it, you have to choose specific solution based on what you want to achieve, what's available etc.
By graphics we really just mean drawing pixels. Things like keyboard and mouse input (which you need for anything interactive like games), loading PNG pictures, playing sounds, loading and displaying 3D models, detecting collisions of virtual objects and so on won't necessarily be covered here, it's all too much to learn at once. We will ONLY be trying to show basic shapes on the screen.
We'll be doing things in simplified ways here, omitting common optimizations, safety checks and so on. Just know that in practice things will be yet a bit more complex.

So, how to actually do graphics? As said, graphics is a super wide topic, there is no silver bullet, all depends on what we really need. Consider the following:

Need to quickly draw something quite basic (e.g. a graph)? Keep it simple and just use ASCII art. You can draw simple pictures to the console with ASCII art, i.e. you emulate real screen pixels with text characters. This is a nice, natural transition from text to graphics when studying programming, so you may start with this. The disadvantage is you can only draw very simple, rough and low resolution pictures, usually without colors, but you can animate them and make your program a little bit interactive. By doing things yourself you'll also get an idea what graphics is about and will better understand why libraries you'll use later work the way they do. A big advantage is that ASCII art graphics can be done without any library (there are libraries like ncurses, but you probably won't need them) and will keep your program quite simple, nice and portable. You can use this for simple visualization, animations, games and so on.
Need to just produce one or two static pictures (e.g. function plot)? Output a picture file. You can make a C program that will simply save a picture to a file which you can open in any image viewer. For this you can use quite simple libraries but it is also possible to load and save simple formats without any libraries at all! You can very easily export bitmap images (e.g. PPM, farbfeld, bmp ...) as well as beautiful vector images (e.g. by exporting SVG) with curves, antialiasing, fancy fonts and so on, you can auto-convert them with other tools to other formats and so on. This will suffice for many things like data visualizations, function plots, photo processing, even 3D rendering, while keeping your program highly portable, i.e. it will be usable everywhere, even on computers without any GUI or screen, it will be much less bloated.
Need a fast, real time interactive program (e.g. a game)? Use a library for that. If you want the "real deal", i.e. interactive, fully colorful high-res graphics, e.g. for a serious game, you'll typically have to use a library -- in C this library is traditionally SDL2 (but there are many alternatives, e.g. SFML, Allegro, SAF, ...). This is a bit more complex, so only go this way if you really must -- you have to install the library, learn to use it and your program will become more bloated, less portable, bigger in size, harder to compile and so on.

We will show an example of each of these approaches further on.

But first let's quickly mention what graphics programming at this level is essentially about, i.e. the kind of "workflow" we'll always try to implement:

The most essential thing is basically to be able to draw a pixel, i.e. set a color of one point in the picture. Once you can draw a single pixel, you can draw anything, just like to build any kind of house you have to be able to lay bricks -- every shape is just some formation of pixels that you can construct with C code: a line is just a series of pixels one next to another, cricle is a curved line, rectangle is just area filled with pixels of some color and so on. So at the beginning we'll just have some way of drawing a single pixel. Typically this can be e.g. a function drawPixel(x,y,color) -- graphic libraries will normally offer you a function like this, letting you draw pixels without actually caring about what magic is going on inside the function. (Sometimes you will also encounter a lower level way in which the library maps a screen to memory and you will draw pixels by literally writing values to memory, i.e. with pointers or arrays.)
With the basic pixel drawing function we'll draw our picture however we want -- if we're using a library, there may be helper functions and of course we can write our own functions too, for example drawLine(fromX,fromY,toX,toY,color), drawText(x,y,text,size,color) and so on. The picture itself is just a virtual canvas, a computer memory holding numbers, typically a two dimensional array whose values are manipulated by the drawPixel function. At this point we are doing nothing else than changing values in memory.
At the end, once drawing is complete, we have to show (present) the picture. This is to say that when we're drawing, the picture isn't actually seen, it is only changing in memory, it is shown to the user only when it's completed, i.e. when we issue a special command such as drawingDone(). Why can't the picture just be shown at all times? In theory it can, but you encounter problems, imagine e.g. a game that quickly redraws the picture on the screen -- here the user would see flickering, he might even see enemies show briefly behind a wall before the wall is actually drawn and so on. So a way to solve this is to do the drawing off screen and only at the end say "now we're done drawing, show the image" (for more details see double buffering).
Also note that usually there is some kind of management around graphic code, i.e. some initialization of the program's window, setting its resolution, allocation of memory for the screen pixels, setting the pixel formats, callbacks and so on. Similarly at the end you often have to clean things up and as many graphic systems are based on events, you have to periodically check events like key presses, window resizes etc. Interactive programs will furthermore have an infinite loop (so called game loop) in which they check events, redraw the screen, wait for a while (to keep the right FPS) and so on. Libraries try to do many thing for you but you have to at least tell them some very basic things. So be prepared for a lot extra code.

Now let's finally do this. We'll set up some basic code for drawing a rectangle and try to draw it with different approaches.

The ASCII approach:

#include <stdio.h>

#define SCREEN_WIDTH 60
#define SCREEN_HEIGHT 25

char screen[SCREEN_WIDTH * SCREEN_HEIGHT]; // our virtual screen

// sets a single pixel at given coordinates
void drawPixel(int x, int y, char pixel)
{
  int index = y * SCREEN_WIDTH + x;

  if (index >= 0 && index < SCREEN_WIDTH * SCREEN_HEIGHT)
    screen[index] = pixel;
}

// presents the drawn picture on the screen
void drawScreen(void)
{
  for (int i = 0; i < 30; ++i) // shift old picture out of view
    putchar('\n');

  const char *p = screen;

  for (int y = 0; y < SCREEN_HEIGHT; ++y)
  {
    for (int x = 0; x < SCREEN_WIDTH; ++x)
    {
      putchar(*p);
      p++;
    }

    putchar('\n');
  }
}

// fills rectangle with given pixel value
void drawRectangle(int x, int y, int width, int height, char pixel)
{
  for (int j = 0; j < height; ++j)
    for (int i = 0; i < width; ++i)
      drawPixel(x + i,y + j,pixel);
}

int main(void)
{
  int quit = 0;
  int playerX = SCREEN_WIDTH / 2, playerY = SCREEN_HEIGHT / 2;

  while (!quit) // main game loop
  {
    drawRectangle(0,0,SCREEN_WIDTH,SCREEN_HEIGHT,'.'); // clear screen with dots
    drawRectangle(playerX - 2,playerY - 1,5,3,'X');    // draw player
    drawScreen();                                      // present the picture

    puts("enter command (w/s/a/d/q):");
    char input = getchar();

    switch (input)
    {
      case 'w': playerY--; break;
      case 's': playerY++; break;
      case 'a': playerX--; break;
      case 'd': playerX++; break;
      case 'q': quit = 1; break;
    }
  }

  return 0;
}

With this we have a simple interactive program that draws a dotted screen with rectangle that represents the player, you can compile it like any other program, it uses no external libraries. User can move the rectangle around by typing commands. There is a main infinite loop (this is the above mentioned game loop, a typical thing in interactive applications) in which we read the user commands and redraw the picture on the screen. Notice we have our basic drawPixel function as well as the drawScreen function for presenting the finished picture, we also have a helper drawRectangle function. The screen array represents our virtual picture (it is declared as one dimensional array but in reality it is treated as two dimensional by the setPixel function). As an exercise you can try to draw other simple shapes, for example horizontal and vertical lines, non-filled rectangles -- if you're brave enough you can also try a filled circle (hint: points inside a circle mustn't be further away from the center than the circle radius).

Now let's try to do something similar, but this time creating a "real picture" made of true pixels, exported to a file:

#include <stdio.h>

#define SCREEN_WIDTH 640   // picture resolution
#define SCREEN_HEIGHT 480

unsigned char screen[SCREEN_WIDTH * SCREEN_HEIGHT * 3]; // screen, 3 is for RGB

// sets a single pixel at given coordinates
void drawPixel(int x, int y, int red, int green, int blue)
{
  int index = y * SCREEN_WIDTH + x;

  if (index >= 0 && index < SCREEN_WIDTH * SCREEN_HEIGHT)
  {
    index *= 3;
    screen[index] = red;
    screen[index + 1] = green;
    screen[index + 2] = blue;
  }
}

// outputs the image in PPM format 
void outputPPM(void)
{
  printf("P6 %d %d 255\n",SCREEN_WIDTH,SCREEN_HEIGHT); // PPM file header

  for (int i = 0; i < SCREEN_WIDTH * SCREEN_HEIGHT * 3; ++i)
    putchar(screen[i]);
}

// fills rectangle with given pixel
void drawRectangle(int x, int y, int width, int height, int red, int green,
  int blue)
{
  for (int j = 0; j < height; ++j)
    for (int i = 0; i < width; ++i)
      drawPixel(x + i,y + j,red,green,blue);
}

int main(void)
{
  drawRectangle(0,0,SCREEN_WIDTH,SCREEN_HEIGHT,128,128,128); // clear with grey
  drawRectangle(SCREEN_WIDTH / 2 - 32, SCREEN_HEIGHT / 2 - 32,64,64,255,0,0);
  outputPPM();
  return 0;
}

Wow, this is yet simpler! Although we have no interactivity now, we get a nice picture of a red rectangle on grey background, and don't even need any library (not even the file library!). We just compile this and save the program output to a file, e.g. with ./program > picture.ppm. The picture we get is stored in PPM format -- a very simple format that basically just stores raw RGB values and can be opened in many viewers and editors (e.g. GIMP). Notice the similar functions like drawPixel -- we only have a different parameter for the pixel (in ASCII example it was a single ASCII character, now we have 3 color values: red, green and blue). In the main program we also don't have any infinite loop, the program is non-interactive.

And now finally to the more complex example of a fully interactive graphic using SDL2:

#include <SDL2/SDL.h> // include SDL library

#define SCREEN_WIDTH 640
#define SCREEN_HEIGHT 480

#define COLOR_WHITE 0xff // some colors in RGB332 format
#define COLOR_RED 0xe0

unsigned char _SDL_screen[SCREEN_WIDTH * SCREEN_HEIGHT]; 
SDL_Window *_SDL_window;
SDL_Renderer *_SDL_renderer;
SDL_Texture *_SDL_texture;
const unsigned char *_SDL_keyboardState;
int sdlEnd;

static inline void drawPixel(unsigned int x, unsigned int y, unsigned char color)
{
  if (x < SCREEN_WIDTH && y < SCREEN_HEIGHT)
    _SDL_screen[y * SCREEN_WIDTH + x] = color;
}

void sdlStep(void)
{
  SDL_Event event;

  SDL_UpdateTexture(_SDL_texture,NULL,_SDL_screen,SCREEN_WIDTH);
  SDL_RenderClear(_SDL_renderer);
  SDL_RenderCopy(_SDL_renderer,_SDL_texture,NULL,NULL);
  SDL_RenderPresent(_SDL_renderer);

  while (SDL_PollEvent(&event))
    if (event.type == SDL_QUIT)
      sdlEnd = 1;

  SDL_Delay(10); // relieve CPU for 10 ms
}

void sdlInit(void)
{
  SDL_Init(0);
  _SDL_window = SDL_CreateWindow("program",SDL_WINDOWPOS_UNDEFINED,
    SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH,SCREEN_HEIGHT,SDL_WINDOW_SHOWN);
  _SDL_renderer = SDL_CreateRenderer(_SDL_window,-1,0);
  _SDL_texture = SDL_CreateTexture(_SDL_renderer,SDL_PIXELFORMAT_RGB332,
    SDL_TEXTUREACCESS_STATIC,SCREEN_WIDTH,SCREEN_HEIGHT);
  _SDL_keyboardState = SDL_GetKeyboardState(NULL);
  SDL_PumpEvents();
}

void sdlDestroy(void)
{
  SDL_DestroyTexture(_SDL_texture);
  SDL_DestroyRenderer(_SDL_renderer); 
  SDL_DestroyWindow(_SDL_window); 
}

int sdlKeyPressed(int key)
{
  return _SDL_keyboardState[key];
}

void drawRectangle(int x, int y, int width, int height, unsigned char color)
{
  for (int j = 0; j < height; ++j)
    for (int i = 0; i < width; ++i)
      drawPixel(x + i,y + j,color);
}

int main(void)
{
  int playerX = SCREEN_WIDTH / 2, playerY = SCREEN_HEIGHT / 2;

  sdlInit();

  while (!sdlEnd) // main loop
  {
    sdlStep(); // redraws screen, refreshes keyboard etc.

    drawRectangle(0,0,SCREEN_WIDTH,SCREEN_HEIGHT,COLOR_WHITE);
    drawRectangle(playerX - 10,playerY - 10,20,20,COLOR_RED); // draw player

    // update player position:
    if (sdlKeyPressed(SDL_SCANCODE_D))
      playerX++;
    else if (sdlKeyPressed(SDL_SCANCODE_A))
      playerX--;
    else if (sdlKeyPressed(SDL_SCANCODE_W))
      playerY--;
    else if (sdlKeyPressed(SDL_SCANCODE_S))
      playerY++;
  }

  sdlDestroy();

  return 0;
}

This program creates a window with a rectangle that can be moved with the WSAD keys. To compile it you first need to install the SDL2 library -- how to do this depends on your system (just look it up somewhere; on Debian like systems this will typically be done with sudo apt-get install libsdl2-dev), and then you also need to link SDL2 during compilation, e.g. like this: gcc -O3 -o graphics_sdl2 -lSDL2 graphics_sdl2.c.

This code is almost a bare minimum template for SDL that doesn't even perform any safety checks such as validating creation of each SDL object (which in real code SHOULD be present, here we left it out for better clarity). Despite this the code is quite long with a lot of boilerplate; that's because we need to initialize a lot of stuff, we have to create a graphical window, a texture to which we will draw, we have to tell SDL the format in which we'll represent our pixels, we also have to handle operating system events so that we get the key presses, to know when the window is closed and so on. Still in the end we are working with essentially the same functions, i.e. we have drawPixel (this time with pixel as a single char value because we are using the simple 332 format) and drawRectangle. This time sdlStep is the function that presents the drawn image on screen (it also does other things like handling the SDL events and pausing for a while to not heat up the CPU).

Where To Go Next

We haven't nearly covered the whole of C, but you should have pretty solid basics now. Now you just have to go and write a lot of C programs, that's the only way to truly master C. WARNING: Do not start with an ambitious project such as a 3D game. You won't make it and you'll get demotivated. Start very simple (a Tetris clone perhaps?). Try to develop some consistent programming style/formatting -- see our LRS programming style you may adopt (it's better than trying to make your own really).

See also supplemental articles at the beginning of this tutorial.

You should definitely learn about common data structures (linked lists, binary trees, hash tables, ...) and algorithms (sorting, searching, ...). As an advanced programmer you should definitely know a bit about memory management. Also take a look at basic licensing. Another thing to learn is some version control system, preferably git, because this is how we manage bigger programs and how we collaborate on them. To start making graphical programs you should get familiar with some library such as SDL.

A great amount of experience can be gained by contributing to some existing project, collaboration really boosts your skill and knowledge of the language. This should only be done when you're at least intermediate. Firstly look up a nice project on some git hosting site, then take a look at the bug tracker and pick a bug or feature that's easy to fix or implement (low hanging fruit).

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