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C - Hello World

Every learner aspiring to become a professional software developer starts with writing a Hello World program in the programming language he/she is learning. In this chapter, we shall learn how to write a Hello World program in C language.

C Hello, World

Before writing the Hello World program, make sure that you have the C programming environment set up in your computer. This includes the GCC compiler, a text editor and preferably an IDE for C programming such as CodeBlocks.

Example

First step is to write the source code for Hello World program. Open a text editor on your computer. On Windows, open Notepad or Notepad++, enter the following code and save it as hello.c

#include <stdio.h>

int main() {
   /* my first program in C */
   printf("Hello, World! \n");

   return 0;
}

Output

Hello, World!

Let us understand how the above program works in a step-by-step manner.

Step 1

The first statement in the above code is the #include statement that imports the stdio.h file in the current C program. This is called a preprocessor directive. This header file contains definitions of several library function used for stand Input/Output operations. Since we shall be calling the printf() function which is defined in it, we need to include it in the first step.

Step 2

Every C program must contain main() function. The main() function in this program prints Hello World message on the console terminal. Inside the main() function, we have inserted a comment statement that is ultimately ignored by the compiler, and is for the documentation purpose. The next statement calls the printf() function. In C, every statement must terminate with semicolon symbol, failing which the compiler reports an error.

The printf() function, imported from stdio.h library file, echoes the Hello World string to the standard output stream, in case of Windows it is the Command prompt terminal and in case of Linux it is the Linux terminal.

In C, every function needs to have a return value. In C, a function must have a return value. If the function doesn’t rerun anything its value is void. In the example above, the main() function has int as its return value. Since the main() function doesn’t need to return anything, it is defined to return an integer 0. The return 0 also indicates that the program has been successfully compiled and run.

Step 3

Next, we need to compile and build the executable from the source code (hello.c).

If using Windows, open the command prompt in the folder in which hello.c has been saved. The following command compile the source code.

gcc -c hello.c -o hello.o

The -c option specifies the source code file to be compiled. This will result in an object file with the name hello.o if the C program doesn’t have any errors. If it contains errors, they will be displayed. For example, if we forget to put the semicolon et the end of the printf() statement, the compilation result shows −

helloworld.c: In function 'main':
helloworld.c:6:30: error: expected ';' before 'return'
   printf("Hello, World! \n")
                             ^
                             ;
helloworld.c:8:4:
   return 0;

To build an executable from the compiled object file, use the following command −

gcc  -o hello.exe hello.o

The hello.exe is now ready to eb run from the command prompt, that displays Hello World message in the terminal.

C:\Users\user>hello
Hello, World!

On Ubuntu Linux, the object file is first given executable permission before running it by prefixing ./ to it.

$ chmod a+x a.o
$ ./a.o

You can also use an IDE such as Code::Blocks to enter the code, edit, debug and run the Hello World program more conveniently.

Using Code::Blocks

Code::Blocks is one the most popular IDEs for C/C++ development. Install it if you have not already done and open it. Create a new file from the File menu, enter the following code and save it as hello.c

Example

#include <stdio.h>

int main() {
   /* my first program in C */
   printf("Hello, World! \n");

   return 0;
}

Output

Hello, World!

Choose Build and Run option from the Build menu as shown below −

Build Menu

You can also use the F9 shortcut for the same. If the program is error-free, the Build Log tab shows the following messages −

gcc.exe   -c C:\Users\mlath\hello.c -o C:\Users\mlath\hello.o
gcc.exe  -o C:\Users\mlath\hello.exe C:\Users\mlath\hello.o   
Process terminated with status 0 (0 minute(s), 0 second(s))
0 error(s), 0 warning(s) (0 minute(s), 0 second(s))
 
Checking for existence: C:\Users\mlath \hello.exe
Executing: '"C:\Program Files\CodeBlocks/cb_console_runner.exe" "C:\Users\mlath\hello.exe"' (in 'C:\Users\mlath\Documents')

In a separate command prompt window the output will be displayed −

Hello, World!

Process returned 0 (0x0)   execution time : 0.236 s
Press any key to continue.

If the code contains errors, the build log tab echoes them. For instance, if we miss the trailing semicolon in the printf() statement, the log will be as below −

Build Messages

You can use any other IDE also to run the C program. You will need to follow the documentation of the respective IDE for the purpose.

Running the Hello World successfully also confirms that the C programming environment is working properly on your computer.

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