C Tutorial



C Tutorial

Define And include in C

C is a statically typed, high-level, general-purpose language that provides a broad range of features, such as low-level access to memory, a simple set of keywords, and a clean style. It is known for its efficiency, versatility, and flexibility, which is why it's widely used in the development of software applications. Among its core features are the #define and #include preprocessor directives. 

This article will explore these two directives, their roles, and how they function.

Introduction to Define and Include in C

In order to simplify programming tasks and improve readability, the C programming language incorporates various tools and features. Two of these tools are the preprocessor directives: #define and #include.

The preprocessor is a program invoked by the compiler that manipulates the source code before the actual compilation starts. It operates under a set of directives, indicated by the # symbol. Among these directives are #define and #include, which perform unique roles.

The #define directive is used for creating symbolic constants and macros. Symbolic constants make programs more modifiable and comprehensible using representative names instead of hard-coded values. For example, we can define the constant PI as #define PI 3.14159. Conversely, macros are like mini-functions that expand through the preprocessor and can take arguments and return values, differing from regular functions.

The #include directive instructs the preprocessor to incorporate the content of another file into the current file. This other file is typically a header file (with a .h extension) containing function prototypes, macro definitions, and other declarations. With the #include directive, code reuse, modularity, and organisation are facilitated, as frequently used functions and macros can be defined in one place (in a header file) and included wherever required.

To sum up, #define and #include fundamental aspects of the C programming language that enhance code readability, maintainability, and organisation. They allow programmers to define symbolic names for hard-coded values, create macro functions for repetitive tasks, and incorporate the contents of other files, thereby promoting code reuse and modularity.

Further, conditional compilation is also made possible using the above-mentioned concepts. 

Conditional compilation is a feature provided by the C preprocessor, allowing portions of the code to be compiled conditionally. This can be incredibly useful in various scenarios, such as platform-specific programming, debugging, and writing header files.

Conditional compilation is controlled by the preprocessor directives #if, #ifdef, #ifndef, #else, #elif, and #endif.

  • #if: This directive begins a conditional block. The block will be included in the compilation if the following condition is true. The condition must be a constant expression.

  • #ifdef: This directive begins a conditional block that will be included in the identifier that follows has been #defined.

  • #ifndef: The opposite of #ifdef, this block will be included if the identifier has not been #defined.

  • #else: This directive is used within a conditional block to start an alternative block that is compiled if the original #if, #ifdef, or #ifndef condition is false.

  • #elif: Stands for "else if". It can be used to chain multiple conditions.

  • #endif: This directive ends a conditional block.

Here's an example demonstrating the use of conditional compilation:

#define DEBUG

int main() {
    #ifdef DEBUG
        printf("Debugging information...\n");

    printf("Program output...\n");

    return 0;

In this example, the printf statement within the #ifdef...#endif block will only be compiled and executed if DEBUG is defined. If you comment out the #define DEBUG line, the debugging information will not be printed.

What is #define in C?

#define is a preprocessor directive in C that defines symbolic constants or macros. It's used to make the program more readable and maintainable.

Syntax of #define in C

The general syntax of #define in C function is as follows:

#define identifier replacement

#define in C Example

Here’s an example code in C that uses #define: 

#define PI 3.14159
#define SQUARE(X) ((X)*(X))

int main(){
    float radius = 2.0, area;
    area = PI * SQUARE(radius);
    printf("Area of the circle is: %.2f", area);
    return 0;

Uses of #define keyword in C

The #define directive in C is primarily used for defining symbolic constants and creating macro functions.

1. Defining Constants: Using #define to assign a name to a constant value is a common practice that improves code readability. When a value is used frequently in a program, it is recommended to #define it as a symbolic constant.

For instance, the mathematical constant pi is frequently used in circle calculations. Instead of typing out 3.14159 every time we need to use pi, we can define it as a symbolic constant at the beginning of our program:

#define PI 3.14159

Now we can use PI anywhere in our program where we'd use 3.14159:

double circumference = 2 * PI * radius;
double area = PI * radius * radius;

2. Creating Macro Functions: Another powerful use of #define is to create macro functions. These are like regular functions, but they're expanded by the preprocessor instead of being called at runtime.

This can lead to more efficient code, but macro functions can also be tricky to use correctly because they're expanded textually. This means you must be careful with operator precedence and side effects.

Here's an example of a define macro in C function that squares a number:

#define SQUARE(x) ((x)*(x))

int main() {
    int num = 5;
    int square = SQUARE(num);
    printf("The square of %d is %d\n", num, square);
    return 0;

In this example, SQUARE(x) is a macro function that squares x. Because macros are expanded textually, putting parentheses around x in the macro definition is important to ensure correct operator precedence. This is a common pitfall with macro functions. 

3. Conditional Compilation: The #define directive can also be used in conjunction with #ifdef, #ifndef, #if, #else, and #elif to control which parts of the program are compiled. This can be useful for creating debug builds, platform-specific code, and more.

For example, we could create a debug build of our program by defining a DEBUG symbol and then using #ifdef to conditionally compile debugging information:

#define DEBUG

int main() {
    #ifdef DEBUG
        printf("Debug build\n");
    // Rest of the program...

Overall, #define is a flexible tool that can help make your code more readable, efficient, and configurable.

What is #include in C?

#include is another preprocessor directive in C. It is used to include the contents of another file in the current file. While writing a C program, you are by default supposed to include some important header files as per your use – especially stdio.h.

Syntax of #include in C

The general syntax of #include is as follows:

#include <file>


#include "file"

Example of #include in C

Here’s an example code in C that makes use of #include: 

#include <stdio.h>

int main(){
    printf("Hello, World!");
    return 0;

In this example, #include <stdio.h> tells the preprocessor to include the standard I/O library (stdio.h), which contains functions such as printf() and scanf().

Uses of #include in C

The #include directive in C is used for inserting the contents of one file into another during the preprocessing phase. This is incredibly useful for various reasons:

1. Including Standard Libraries: Perhaps the most common use of #include is to include standard library header files in your program. These files, which have the .h extension, contain function declarations and macro definitions that are part of the C standard library.

For example, if you want to use the printf() function to print to the console, you would need to include the stdio.h file:

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;

In this case, #include <stdio.h> tells the preprocessor to include the contents of stdio.h in your program. This file contains the declaration of printf(), among other things.

2. Including User-Defined Header Files: #include can be used to include your own header files. This is very useful for larger programs, as it allows you to separate your program into multiple files.

For instance, suppose you have a header file myfunctions.h, which contains the declaration of a function void myFunction();. You can include this file in your program and then use myFunction():

#include "myfunctions.h"

int main() {
    return 0;

3. Modularity and Code Reusability: By placing commonly used function declarations, macro definitions, and other code in a header file, you can #include it in multiple source files. This not only makes your code more organised but also allows you to reuse code across multiple projects.

Remember that the #include directive simply copies the contents of the specified file into the current file. The file to be included is specified using either angle brackets <> (for system header files) or double quotes "" (for user-defined header files).

What is the Difference between #define vs #include in C

Though both #define and #include in C are preprocessor directives, they serve different purposes and have different uses.

#define is a preprocessor directive for defining symbolic constants and macros, improving code readability and reusability.

On the other hand, #include is a preprocessor directive used to insert file contents, typically header files, into another file, allowing access to functions and macros defined in those files.

In essence, #define is about creating symbolic constants and macros, whereas #include is about bringing in external code (in the form of header files) to use in your program.

Here is a comparison table summarising the key differences:



Used for defining symbolic constants and macros

Used for including header files

Replaces defined identifiers with their corresponding replacement text

Inserts the entire contents of one file into another

Does not involve any sort of file handling

Involves file handling

Can be used to conditionally compile sections of code

Cannot be used for conditional compilation

Does not increase the executable size, as the preprocessor deals with it

Including unnecessary files can increase the executable size

Syntax: #define identifier replacement_text

Syntax: #include <file> or #include "file"


The #define and #include in C programming are invaluable tools, and understanding their purposes and differences is crucial for writing efficient and maintainable code. They allow you to define symbolic names for constant values, create reusable macros, and incorporate code from external files, enhancing modularity and reusability.

While this article provided a solid foundation, mastering the concepts of C programming and advancing your skills requires practice and in-depth study. If you're looking to enhance your knowledge to the next level, check out upGrad's Master of Science in Computer Science program, delivered in collaboration with Liverpool John Moores University. This course offers in-depth coverage of topics like software engineering, data science, AI, and machine learning, providing the skills you need to advance in the fast-paced tech industry. 


Q1: Can I use a #define macro in a #include statement?

Yes, it's possible to use a #define macro within an #include statement. For instance, consider the following:

#define HEADER "my_header.h"
#include HEADER

Q2: How are #define constants different from const in C?

#define constants are processed by the preprocessor and do not have a memory location, while const variables do have a memory location. const variables also have a type, whereas #define constants don't.

Q3: Can #define be used for function declaration?

#define can be used to create macro functions, but it should not be used for actual function declarations. Macros are expanded by the preprocessor, and this can lead to unexpected behaviour if not used carefully.

Q4: What is the difference between #include "file" and #include <file>?

The #include "file" form looks for the specified file in the current directory first, then the standard library directories. In contrast, #include <file> looks for the file only in the standard library directories.

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