Kiran Chauhan

GTK4: Chapter 01

Let's open the text-editor with blank window. Quickly save this blank program with name hello.c. C language has .c extension and hello is the name of the program that satisfies the purpose of code, going to written inside of it. After saving this blank program, I have syntax highlighting. Good!

Next, we are going to print hello, world text in terminal. So, we need to write this text within hello.c. Let me do that.

hello, world

But, this hello, world is string, right? In program, you write string inside of double or single quotes. Agree? But in C, you must write string inside of double quotes. So, let me quickly wrap this text with double quotes and make it string.

"hello, world"

Great! This string is not going to print in terminal on its own. We need to invoke a function on it to print and the function is printf(). This is how we use printf() function.

printf("hello, world")

C is one of those languages the required semicolon to complete the statement. In C, semicolon is not optional but required. And that way I need to write,

printf("hello, world");

Semicolon at the end. Great. But you know, compiled languages like C is different than interpreted language in terms of execution. In interpreted languages, the execution start from top and execute each statement or expression that comes in flow. But, in compiled language you need to tell the starting point - a point from where the compilation process starts. This is where the main() function comes in picture. Your program must contain one main() function and this is the function from where your program will going to compile.

In order to run our printf() statement, we need to put it inside of main() function this way.

main() {
  printf("hello, world");
}

This indentation inside of function is not required. It is just for readability purpose.

You are going to use curly braces to define block like I did to define main() function block. Whatever statements we write inside of this curly braces, are considered part of the main() function.

After compilation of this main() function, it returns something. It returns an integer with 0 or greater than 0. Here, 0 means successful compilation and any other value than 0 mean something goes wrong with your program! Let's add the type signature before the main() function to indicate, what this main() is going to be written and also write return 0 explicitly.

int main() {
  printf("hello, world");

  return 0;
}

Yes, this return statement should be at the end of main() function.

In C, you don't get a single function by default and printf() is not an exception. You need to include a library in which this function is written. The library or header file is stdio.h and its standard header file that comes with C. You can include this file in our program like -

#include<stdio.h>

int main() {
  printf("hello, world");

  return 0;
}

You need to write the include statement at the top of your program and the blank line after this include statement is optional. But, again it is strongly recommended.

That’'s it! This is our hello, world program in C. It's time to execute this program.

Execution

We need a compiler to execute any C program. For this purpose, we are going to use GCC. This is the compiler that comes with most of the GNU/Linux system. A quick --version command will let you know that C is installed on your system or not.

$ gcc --version
gcc (GCC) 8.3.1 20190223 (Red Hat 8.3.1-2)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

If you get similar output, then you have gcc on your system else you need to install. In my case, I've gcc so I can go further and execute this program. The execution of C program done in two steps —

1. Compiles the program,
2. Executes the compiled program.

To compile the program, this command is used.

$ gcc hello.c -o hello

This will compile your hello.c program and after successful completion it will generate output (-o) with name hello. You will get nothing if compilation happen successfully in terminal. In my case, I don't get anything, so that means my program execute successfully and has build hello. A quick ls will show you the same.

$ ls
hello.c hello

In order to execute this hello executable, I need to write:

$ ./hello
hello, world

And we get the output! But, this is a little clumsy. A \n after the hello, world string will help. Let's add that.

#include<stdio.h>

int main() {
  printf("hello, world\n");

  return 0;
}

Let's compile and run the program again.

$ gcc hello.c -o hello && ./hello
hello, world

Here, I've combines both commands into single one using && operator. You will going to use so many terminal commands in this book. So, be ready!