Joe's blog

C was the second programming language I learned. After playing with BASIC for around 4 years, I started programming in C at the age of 15. Till today C remains my favorite programming language. It was only after 3 more years, that I had the experience of using Linux. I was amazed at seeing that there are a lot of programming tools available on Linux. But I had no idea how to compile a simple program that would print out “Hello World”.

This is the first in line of a series of tutorials that I am planning to write on C programming in Linux. We will start off with a simple program that prints out Hello World. It would be nice if you create a directory for C programs you will be doing as part of this tutorial. I have created a directory cprogramming. In the directory open up your favorite editor and write and save the following program as helloworld.c

/* Program to print out Hello World on screen */
#include <stdio.h>
main()
{
         printf("Hello World");
}

Explanation

The first line is a comment. It is meant for documentation purpose only. The next line is called is a preprocessor directive. It tells the compiler that we are using functions defined in the file stdio.h in the program. Many basic input/output functions are declared in stdio.h .

Next is the main() function. Every executable C program must have a main() function and execution of the program starts in main() function. What lies in between the curly brackets {} is the body of main() function.

The printf(“Hello World”); statement is to print Hello world on the screen.

Compiling and running the program

On the terminal execute the command

gcc helloworld.c

On executing ls command you can find a new file a.out in the directory. It is the executable of helloworld.c . Unless otherwise stated the name of executatble programs generated by the gcc compiler is named as a.out . Lets execute the program by typeing ./a.out on the command line. We can see that the program prints out Hello World

As I had stated earlier every executable program generated by gcc has the default name a.out . We can specify a name for the executable at the time of compiling. At the command line type the following command

gcc -o helloword helloworld.c

-o option of the gcc command specifies the name for the executable file to be generated. Now we have an executable file helloworld in the directoy.

I think thats enough for the first post on C programming in Linux. We have learned how to compile and execute a simple C program in Linux. I will be discussing more advanced topics in the next posts.