Chapter 1: C Basics

C is a general purpose programming language designed and developed by Dennis M. Ritchie et al. in the early 1970's at Bell Labs and is a successor to the B language (also developed by Dennis Ritchie).

C is a widely used language in a lot of systems or embedded programming areas and can compile to machine code that can run in a lot of different computer architectures.

Hello World #

Given the file hello-world.c with the ff. contents:

#include <stdio.h>

int main() {
  printf("Hello world!\n");
}

All programs in C must contain at most one main() function which will serve as the entry point of all execution.

The very first line: #include <stdio.h> describes that the standard I/O library is to be included in the program in which case provides printf() among other things.

To run the C program it needs to be compiled, for example using gcc which should generate an executable:

# Compile the program
gcc ./hello-world.c
# Run the executable
./a.out

By default gcc will compile the program as a.out, if one wants to use a different name for the executable use the -o flag followed by the desired name:

gcc <your-c-file> -o <executable-name>

If no errors come up the program should output Hello world!.

About printf() #

As stated printf() is a function provided from the standard I/O library: stdio.h which prints output to some terminal device.

The first argument is a string of characters that may contain special symbols that can be replaced or provide special meaning such as \n (new line) and \t (tab).

Placeholder symbols start with the character % and denotes to printf() that it should substitute it to whatever value is put in the second or so places of arguments in the function call, for example:

#include <stdio.h>

int main() {
  printf("The number %d", 5);
}

Here, the %d is a placeholder symbol that will print an integer (in this case 5).

The number of placeholder symbols in the first argument of printf() should have the same corresponding number of values to be replaced with and must also have the same type ie a %d should receive an integer value.

Other placeholder symbols are as follows:

Declarations #

Variables #

Variables are defined by providing a type and then the identifier for that variable, for example:

int main() {
  int my_number = 0;
}

Defines a variable my_number with a type int and assigns it with a value 0.

Symbolic Constants #

Symbolic Constants define symbols that are replaced when the program is compiled:

#include <stdio.h>

#define PI 3.14

int main() {
  printf("π equal %d", PI);
}

Control Flow #

C provides common control flow statements such as if/else if/else, while/do while, for, and switch case.

Functions #

A function in C is defined by first declaring the return type then the identifier of the function, and then parameters, and lastly the function body:

#include <stdio.h>

int square(int n) {
  return n * n;
}

int main() {
  printf("2^2 = %d", square(2));
}

Basic I/O #

The standard I/O library also provides a way to read inputs such as getting a character from the user and printing that character, these are getchar() and putchar() respectively.

#include <stdio.h>

int main() {
  int c = getchar();
  putchar(c);
}

In this example:

1. int c = getchar() will wait for an character input from the user and once this is fulfilled will assign the value to the variable c. Note that getchar() returns the integer value of the character.
2. putchar(c) will just print the character of whatever value c is.

Arrays #

Arrays are notations that provide a sequence of data:

int main() {
  char s[5];
}

In this example, s is a char array that can contain at most 5 items in it.

Sharing definitions using extern #

To share definitions to other C files or to just provide a globally accessible definition use the extern keyword to that definition.