C Preprocessor Directives

In C programming, the preprocessor is a program that process the source code before the actual compilation begins. It uses preprocessor directives are commands that instruct the preprocessor to perform specific actions. These directives start with the # symbol.

List of Preprocessor Directives

The following table lists some commonly used preprocessor directives available in the C programming language:

#define

In C, the #define preprocessor directive is used to define the macros and symbolic constants. The macros are the identifiers defined by #define which are replaced by their value before compilation.

Example

#include <stdio.h>

// Defining a macro for PI
#define PI 3.14159

int main(){
    
    // Using the PI macro to calculate
    double r = 8.0;
    10double area = PI * r * r;
    
    printf("%f\n", a);
    return 0;
}

201.061760

#include

#include preprocessor directive is used to include the contents of a specified file into the source code before compilation. It allows you to use functions, constants, and variables from external libraries or header files. There are two types:

#include <file_name>
#include "filename"

Here, file inclusion with double quotes ( ” ” ) tells the compiler to search for the header file in thedirectory of source filewhile <> is used for system libraries.

Example

// Including standard input output library
// using its header file
#include <stdio.h> 

int main(){
  
    // Using standard library functions
    printf("Hello, Geek!\n");

    return 0;
}

Hello, Geek!
Sum: 15

#if, #ifdef, #else, #elif, #endif

The above directives are Conditional Compilation directives that help to compile a specific portion of the program or let us skip compilation of some specific part of the program based on some conditions.

#if, #else and #elif

These directives work together to control which parts of the program get compiled based on certain conditions.

• If the condition after the #if is true, the lines after it will be compiled.
• If not, it checks the condition after associated #elif. If that's true, those lines will be compiled.
• If neither condition is true, the lines after #else will be compiled.

Example:

#include <stdio.h>

int main() {
    #define VALUE 2  

    // Check if VALUE is greater than 3
    #if VALUE > 3  
        printf("Value is greater than 3\n");

    // Check if VALUE is 3 
    #elif VALUE == 3  
        printf("Value is 3");

    // If neither conditions are true, this
    // block will execute
    #else  
        printf("Value is less than or equal to 2");
    #endif

    return 0;
}

Value is less than or equal to 2

Note: the entire structure of #if, #elif and #else chained directives ends with #endif.

#ifdef

The #ifdef (if defined) directive checks whether a macro is defined. If it is, the code within the #ifdef block is included in the program.

Example

#include <stdio.h>
int main() {
    
#define DEBUG

// Check if DEBUG is defined
#ifdef DEBUG
    printf("Debugging is enabled");
#endif

    return 0;
}

Debugging is enabled

#ifndef

The #ifndef (if not defined) directive checks if a macro is not defined. If it is not defined, the code inside the #ifndef block is included.

Example:

#include <stdio.h>
int main() {

// Check if DEBUG is not defined
#ifndef DEBUG
	printf("Debugging is not enabled");
#endif
	return 0;
}

Debugging is not enabled

#line

The #line directive allows you to specify a line number and filename to be used by the compiler. This is often used in generated code or debugging.

Example

#include <stdio.h>

// Define macro that prints current line number and filename
#define PrintLineNum printf("Line number is %d in file named %s\n", __LINE__, __FILE__)

int main() {
    PrintLineNum;

    // Change line number to 20 and filename to "main.c"
    PrintLineNum;

    // Change line number to 30 and filename to "index.c"
    #line 30 "index.c"
    PrintLineNum;

    return 0;
}

Line number is 5 in file named ./Solution.c
Line number is 20 in file named main.c
Line number is 30 in file named index.c

Line number that will be assigned to the next code line. The line numbers of successive lines will be increased one by one from this point on. "filename" - optional parameter that allows to redefine the file name that will be shown.

#error

The #error directive generates a compilation error with a custom message. This is useful for detecting and reporting certain conditions during preprocessing. This immediately terminates the compilation.

Example

#include <stdio.h>

#ifndef GeeksforGeeks

// Show error message
#error "GeeksforGeeks not found!"
#endif

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

Output

error: #error "GeeksforGeeks not found!"

#pragma - Pragma Directive

The #pragma directive is a special purpose directive that is used to turn on or off some features. #pragma also allows us to provide some additional information or instructions to the compiler. It is compiler-specific i.e., the behaviour of pragma directive varies from compiler to compiler.

Commonly used pragma directives are:

• #pragma message: used at compile time to print custom messages.
• #pragma once: to guard the header files i.e the header file must be included only once.
• #pragma warning: to enable or disable the warnings.

Example

#include <stdio.h>

// not defining gfg to trigger pragma message
// #define GeeksforGeeks

int main(){
#ifndef GeeksforGeeks
#pragma message(" GfG is not defined.")
#endif
    printf("Hello geek!\n");
    return 0;
}

Output

#pragma message:  GfG is not defined.