What is Preprocessor in C ?

In C, a preprocessor is a part of the C compiler that performs text manipulations before the actual compilation process begins. It is responsible for preprocessing source code before it is sent to the compiler. Preprocessors are activated by special commands, called preprocessor directives, that start with a ‘#’ symbol. Preprocessors are used for tasks such as including header files, conditional compilation, macro expansion, and more. They help in making the code more maintainable and portable.

Preprocessors in C

Here is a table of some commonly used preprocessor directives in C, along with their descriptions:

1. #include

#include <stdio.h>

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

Output:

Hello, World!

Explanation:

The #include directive is used to include the standard input-output library (<stdio.h>) in the code. This allows you to use functions like printf in your program.

2. #define

#include <stdio.h>

#define PI 3.14159265

int main() {
    float radius = 5.0;
    float area = PI * radius * radius;
    printf("Area of the circle: %f\n", area);
    return 0;
}

Output:

Area of the circle: 78.539818

Explanation:

#define is used to create a macro. In this example, we define a macro PI with a value of 3.14159265. The macro is then used in the calculation of the area of a circle.

3. #undef

#include <stdio.h>

#define DEBUG

int main() {
    #ifdef DEBUG
        printf("Debug mode is enabled.\n");
    #endif

    #undef DEBUG

    #ifdef DEBUG
        printf("Debug mode is still enabled.\n");
    #else
        printf("Debug mode is now disabled.\n");
    #endif

    printf("Program executed.\n");
    return 0;
}

Output:

Debug mode is enabled.
Debug mode is now disabled.
Program executed.

Explanation:

#undef is used to undefine a previously defined macro. In this example, we define the DEBUG macro initially and check if it’s defined. After #undef DEBUG, we check again, and it’s no longer defined.

4. #ifdef

#include <stdio.h>

#define DEBUG

int main() {
    #ifdef DEBUG
        printf("Debug mode is enabled.\n");
    #endif

    printf("Program executed.\n");
    return 0;
}

Output:

Debug mode is enabled.
Program executed.

Explanation:

#ifdef checks if the macro DEBUG is defined. In this case, it is defined, so the code within the conditional block is included in the compilation.

5. #ifndef

#include <stdio.h>

#ifndef VERBOSE
    #define VERBOSE
#endif

int main() {
    #ifdef VERBOSE
        printf("Verbose mode is enabled.\n");
    #endif

    printf("Program executed.\n");
    return 0;
}

Output:

Verbose mode is enabled.
Program executed.

Explanation:

#ifndef checks if the macro VERBOSE is not defined. In this case, it’s not defined, so the #define VERBOSE directive defines the macro before the code is compiled.

6. #if

#include <stdio.h>

#define MY_VALUE 42

int main() {
    #if MY_VALUE > 50
        printf("Value is greater than 50.\n");
    #else
        printf("Value is not greater than 50.\n");
    #endif

    return 0;
}

Output:

Value is not greater than 50.

Explanation:

#if allows conditional compilation based on an expression. In this example, it checks if the macro MY_VALUE is greater than 50 and executes the appropriate code block.

7. #else

#include <stdio.h>

#define DEBUG

int main() {
    #ifdef DEBUG
        printf("Debug mode is enabled.\n");
    #else
        printf("Debug mode is disabled.\n");
    #endif

    printf("Program executed.\n");
    return 0;
}

Output:

Debug mode is enabled.
Program executed.

Explanation:

#else provides an alternative branch in a conditional compilation block. If the condition specified with #ifdef is not met, the code within the #else block is executed.

8. #elif

#include <stdio.h>

#define VERSION 2

int main() {
    #if VERSION == 1
        printf("Version 1 is selected.\n");
    #elif VERSION == 2
        printf("Version 2 is selected.\n");
    #else
        printf("Unknown version is selected.\n");
    #endif

    return 0;
}

Output:

Version 2 is selected.

Explanation:

#elif is used for else-if branching within a conditional compilation block. It checks multiple conditions and executes the code block associated with the first true condition.

9. #endif

#include <stdio.h>

#define MY_VALUE 42

int main() {
    #if MY_VALUE > 50
        printf("Value is greater than 50.\n");
    #else
        printf("Value is not greater than 50.\n");
    #endif

    return 0;
}

Output:

Value is not greater than 50.

Explanation:

#endif marks the end of a conditional compilation block. In this example, it ends the block started with #if and #else.

10. #error

#include <stdio.h>

#ifndef CONFIG
    #error "CONFIG macro is not defined."
#endif

int main() {
    printf("Program executed.\n");
    return 0;
}

Output:

error: #error "CONFIG macro is not defined."

Explanation:

The #error directive generates a compilation error message along with the specified error message when the condition is not met. In this case, it checks if the CONFIG macro is defined, and if not, it raises a compilation error.

11. #pragma

#include <stdio.h>

#pragma startup myStartupFunction
#pragma exit myExitFunction

void myStartupFunction() {
    printf("Startup function called.\n");
}

void myExitFunction() {
    printf("Exit function called.\n");
}

int main() {
    printf("Main function executed.\n");
    return 0;
}

Output:

Startup function called.
Main function executed.
Exit function called.

Explanation:

#pragma is used to provide compiler-specific instructions or options. In this example, it specifies functions to be called at startup and exit of the program.

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