less_retarded_wiki/c_tutorial.md

C Tutorial

This is a relatively quick WIP C tutorial.

You should probably know at least the completely basic ideas of programming before reading this (what's a programming language, source code, command line etc.). If you're as far as already knowing another language, this should be pretty easy to understand.

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.
• 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 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.

Programming in C works like this:

1. You write a C source code into a file.
2. 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.
3. 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 can format 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 withing 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.

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. 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

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 hold, 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.

A note on 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 means false and anything non-0 means true. Even comparison operators like < and > are technically arithmetic, they compare numbers and yield either 1 or 0.

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 < 10 (not i <= 10).

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

Functions are extremely important, no program besides the most primitive ones can be made without them.

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 and 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 and 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:

2
4
8
16

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 function 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), it 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 return 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 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 divisord 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.

Header Files, Libraries, Compilation

Advanced Data Types

Macros

Pointers

More on Functions

Dynamic Allocation

Debugging

Advanced Stuff

Under The Hood