C Tutorial



C Tutorial

Macros in C

Macros in C are powerful tools that allow developers to define reusable code snippets. They are defined using preprocessor directives and primarily used for code substitution and generation. Macros provide a convenient way to write concise and efficient code, improving the readability and maintainability of C programs. 

In this article, we will explore the concept of macros in C, their types, and how they can be used effectively in programming.

Introduction to Macros in C Language

Macros in C are a feature of the C preprocessor. The preprocessor is a part of the C compiler that performs various transformations on the source code before it is compiled. Macros, defined using preprocessor directives, are used to replace specific code snippets with predefined values or expressions during the compilation process.

What are Macros in C language?

In C programming, macros are defined using the #define directive. The syntax for defining a macro is as follows:

#define MACRO_NAME value

When the C compiler encounters a macro in the source code, it directly substitutes the macro with its specified value or expression. This substitution happens before the compilation takes place, meaning that macros are not evaluated at runtime but at compile time.

Macros can be used for simple textual substitutions or for defining complex code blocks. They are commonly used to define constants, inline functions, and conditional compilation statements.

Why Do We Use Macros in C?

Macros in C provide several benefits and serve various purposes. Let's explore some of the common use cases of macros along with relevant macros in C examples and explanations:

1. Constants

Macros can be used to define constants in C programs. Instead of using literal values throughout the code, macros provide a symbolic representation, enhancing code readability and maintainability.

#define PI 3.1415
#define MAX_SIZE 100

In the above example, PI represents the value of π, and MAX_SIZE represents the maximum size for an array or buffer. Using macros for constants allows easy modifications if the value needs to be changed in the future.

2. Code Simplification

Macros can simplify complex or repetitive code by replacing it with a concise macro definition. This reduces the amount of code that needs to be written and improves code readability.

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

The SQUARE macro calculates the square of a given value. Instead of writing the multiplication expression (x) * (x) repeatedly, we can simply use SQUARE(x) to achieve the same result. This simplifies the code and makes it more readable.

3. Inline Functions

Macros can be used to define inline functions, eliminating the overhead of function calls and improving performance in certain cases. Inline functions are expanded directly at the point of invocation, saving the time and resources required for function call overhead.

#define MAX(a, b) ((a) > (b) ? (a) : (b))

The MAX macro function compares two values, a and b, and returns the maximum of the two. Using this macro avoids the overhead of a function call and provides a more efficient way to determine the maximum value.

4. Conditional Compilation

Macros are commonly used for conditional compilation, allowing specific sections of code to be included or excluded based on preprocessor directives. This enables code to be customised and adapted for different environments or configurations.

#define DEBUG_MODE

    // Debugging code
    printf("Debug information\n");
    // Release code
    printf("Release information\n");

In the above example, the DEBUG_MODE macro is defined, which enables or disables the debugging code depending on its presence. During compilation, if DEBUG_MODE is defined, the debugging code block will be included; otherwise, the release code block will be included. 

5. Compile-time Computations

Macros in C are evaluated at compile time, allowing for efficient compile-time computations. This can be useful when performing calculations or generating code that doesn't need to be evaluated at runtime.

#define FACTORIAL(n) \
    ({ \
        int result = 1; \
        for (int i = 1; i <= (n); i++) \
            result *= i; \
        result; \

int factorial = FACTORIAL(5);  // Evaluates to 120 at compile time

In the above example, the FACTORIAL macro computes the factorial of a given number. The computation is done at compile time, and the result is substituted directly into the code. This can be especially beneficial for performance-critical applications where runtime computations are undesirable.

What are Preprocessor and Preprocessor Directives in C Programming?

In C programming, the preprocessor is a component of the compiler that performs various transformations on the source code before compilation. It operates on the source code at a textual level and is responsible for handling preprocessor directives. Preprocessor directives are special instructions that start with a # symbol and provide instructions to the preprocessor. 

Let's explore some commonly used preprocessor directives along with relevant code examples:

1. #define

The #define directive is used to define macros. It associates a name with a value or an expression.

#define PI 3.1415
#define MAX(a, b) ((a) > (b) ? (a) : (b))

In the above code, the #define directive is used to define macros. The initial line defines a macro called PI with the assigned value of 3.1415, whereas the subsequent line defines a macro function known as MAX, responsible for returning the maximum value between two given values.

2. #include

The #include directive is used to include the contents of another file into the current source file.

#include <stdio.h>
#include "myheader.h"

In the above code, the #include directive is used to include the standard input/output library stdio.h and a user-defined header file named myheader.h. 

3. #ifdef, #ifndef, #else, #endif

Conditional compilation directives allow including or excluding specific code blocks based on predefined macros or conditions.

#ifdef DEBUG
    // Debugging code
    // Release code

The #ifdef directive checks if the macro DEBUG is defined, including the debugging code block if true. The #else directive specifies an alternative code block if the condition is not met, and the #endif directive marks the end of the conditional compilation block.

4. #pragma

The #pragma directive provides additional instructions to the compiler. Its functionality is implementation-dependent.

#pragma omp parallel
    // Parallelized code

In the above code, the #pragma directive is used to specify a parallel execution environment using OpenMP. This directive instructs the compiler to parallelise the code block enclosed within the curly braces.

Preprocessor directives modify source code before compilation, enabling code modification, macro definition, conditional compilation, and compiler-specific instructions, enhancing C program flexibility.

Preprocessor directives are processed before compilation, replacing directives with instructions or code during preprocessing, leading to a modified source code for compilation.

Types of Macros in C Language

C programming comprises two distinct types of macro in C. These include:

Object-like Macros: These macros are defined using the #define directive and are primarily used for textual substitution. They are similar to constants and are often used to define symbolic names or constants.

#define PI 3.1415
#define MAX(a, b) ((a) > (b) ? (a) : (b))

In the above example, PI is an object-like macro representing the value of π, while MAX is a macro function that returns the maximum of two values.

Function-like Macros: Function-like macros are defined using the #define directive and are used to define inline functions. They can take arguments and can generate code based on those arguments.

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

The SQUARE macro calculates the square of the given argument by performing the multiplication.

Predefined Macros in C Language

In addition to user-defined macros, C provides predefined macros offering valuable information about the program and compilation environment, accessed during compilation. 

Let's explore some common predefined macros with their explanations and code examples:

1. __FILE__

The __FILE__ macro represents the current source file name. It expands to a string literal that contains the name of the file being compiled.

printf("Current file: %s\n", __FILE__);

The above code will display the name of the current source file at runtime. This macro can be useful for debugging and logging purposes, providing information about the source file being executed.

2. __LINE__

The __LINE__ macro represents the current line number in the source file. It expands to an integer literal that indicates the line number at which the macro appears.

printf("Current line: %d\n", __LINE__);

When executed, the above code will display the line number where the printf statement is located. This macro is often used for debugging and error reporting, providing information about the line of code being executed.

3. __DATE__

The __DATE__ macro represents the date of compilation. It expands to a string literal in the format "MMM DD YYYY".

printf("Compilation date: %s\n", __DATE__);

When the program is compiled, the above code will display the compilation date. This macro can help with version and document purposes, providing information about when the code was compiled.

4. __TIME__

The __TIME__ macro represents the time of compilation. It expands to a string literal in the format "HH:MM:SS".

printf("Compilation time: %s\n", __TIME__);

When executed, the above code will display the time the program was compiled. This macro can be useful for tracking the build time or for time-based operations during program execution.

5. __cplusplus

The __cplusplus macro is defined in C++ programs and indicates the version of the C++ standard being used. In C programs, this macro is not defined.

#ifdef __cplusplus
    printf("C++ program\n");
    printf("C program\n");

In the above code, the presence of __cplusplus is checked to determine whether the code is being compiled as a C++ program or a C program. This macro allows conditional compilation based on the programming language.

Predefined macros enhance C program flexibility and adaptability by providing valuable information during compilation, enabling developers to include or exclude code based on conditions or gather compilation environment details.


Macros and preprocessor directives in C programming offer powerful tools for code substitution, inclusion, conditional compilation, and compiler-specific instructions, improving code readability, performance, and customisation.

Understanding the concept of macros, preprocessor directives, and their various applications is essential for writing efficient and flexible C code. By utilising macros and preprocessor directives effectively, developers can enhance their productivity, optimise their code, and achieve better control over program behaviour.

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Q: What is a macro in C with an example?

A macro in C is a code snippet defined using the #define directive that is replaced with a specified value or expression during compilation. Here's an example:

#define PI 3.1415

In this example, the PI macro is defined with the value 3.1415, and any occurrence of PI in the code will be replaced with 3.1415 during compilation.

Q: What are the types of macros in C?

There are two main types of macros in C: object-like and function-like macros. Object-like macros are primarily used for textual substitution, while function-like macros are used to define inline functions that can take arguments and generate code based on those arguments.

Q: What are predefined macros in C?

Predefined macros in C are macros already defined by the compiler. They provide information about the program and the environment during compilation. Examples of predefined macros include __FILE__, __LINE__, __DATE__, and __TIME__.

Q: How does macro expansion work in C?

Macro expansion in C is the process by which the C preprocessor replaces macro invocations with their corresponding definitions during compilation. When the compiler encounters a macro in the source code, it replaces the macro with the specified value or expression before compilation.

Q: How are macro functions different from regular functions in C?

Macro function in C are inline functions expanded during preprocessing, offering performance benefits by avoiding the function call overhead. However, they lack type checking, potentially leading to unexpected behaviour. Regular functions, separate code blocks called at runtime, provide type safety and other features.

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