The very first C program to write is the one to print "Hello World!" to the screen.
We create a folder with the following file in it:
examples/hello_world/hello_world.c
#include <stdio.h>
int main() {
printf("Hello World!\n");
}
On Linux we will use the gcc compiler. There are others, but as I understand this is the most popular compiler.
Install gcc on Ubuntu Linux
$ sudo apt update
$ sudo apt install -y gcc
Compile and run
Before we can run the program we need to compile it. We can do it with the following command:
$ gcc hello_world.c
This will create a file called a.out and make it executable. We can then run it:
$ ./a.out
Hello World!
Compile to a specific name
You might not like the default name of a.out. You can tell gcc how to call the generated binary file using the -o (output) flag:
$ gcc -o hello hello_world.c
This will create a file called hello which we can run as ./hello.
On Linux executables don't need to have an extension.
Explanation of the source code
Every C program must have a function called main that is expected to return an integer int. We are not required to actually return anything, but this is
how we need to define the function. This value will become the exit code of our program.
C uses the C-like syntax (what a surprise here), using curly braces to define blocks of code.
The indentation and in general the layout of the code does not matter to the C compilers, but they matter a lot to the human who will read the code so we will use indentation.
There is a never ending debate on the indentation style (spaces or tabs, how many space) and where one should put the opening and closing curly braces.
For now I'll use the style I used to use for years other languages.
The C language is very simple, it can't even print to the screen on its own. We need to use a library to be able to print to the screen. The line
#include <stdio.h>
includes the stdio (Standard Input and Output) library.
This allows us to use the printf function to print to the screen.
We put the text (string) in double-quotes and we use the \n special symbol to tell C to print a newline.
