C Program Structure

Before we learn the basic building blocks of C, let's first look at the structure of a minimal C program, which we can use as a reference in the following chapters.

C Hello World Example

C programs mainly consist of the following parts:

  • Preprocessor directive
  • Function
  • Variable
  • Statements & expressions
  • Comments

Let's look at a simple piece of code that can output the word "Hello World":

Example

#include <stdio.h> int main() { /*My First C Program*/ printf("Hello, World! \n"); return 0; }

Next, let's explain the above program:

  1. The first line of the program#include <stdio.h>It is a preprocessor directive that tells the C compiler to include the stdio.h file before actual compilation.
  2. Next lineint main()It is the main function; the program starts executing from here.
  3. The next line /*...*/ will be ignored by the compiler; this is where the program comments go. They are called program comments.
  4. Next lineprintf(...)It is another function available in C that displays the message "Hello, World!" on the screen.
  5. Next linereturn 0;It terminates the main() function and returns the value 0.

Compiling & executing C programs

Next, let's see how to save the source code in a file, and how to compile and run it. Here are the simple steps:

  1. Open a text editor and add the above code.
  2. Save the file ashello.c。
  3. Open the command prompt and go to the directory where the file is saved.
  4. Typegcc hello.c, press Enter, and compile the code.
  5. If there are no errors in the code, the command prompt will jump to the next line and generatea.out (Windows generates a.exe)Executable file.
  6. Now, typea.outto execute the program (under Windows, typea.exe）。
  7. You can see the following displayed on the screen:"Hello World"。
$ gcc hello.c
$ ./a.out
Hello, World!

Make sure your path already includes the gcc compiler, and ensure that you run it in the directory containing the source file hello.c.

If there are multiple C source code files, the compilation method is as follows:

$ gcc test1.c test2.c -o main.out
$ ./main.out

test1.c and test2.c are two source code files.

other extensions