C Macros

C Macros and Preprocessor Directives

The C Preprocessor is not part of the compiler itself, but rather a separate step that happens before compilation. It processes directives (commands starting with #) to include files, conditionally compile code, and define Macros.

A Macro is a fragment of code that has been given a name. Whenever the name is used, it is replaced by the contents of the macro.

1. Object-Like Macros

The most common use of #define is to create named constants. This makes your code highly readable and easy to modify.

#include <stdio.h>

#define PI 3.14159
#define MAX_USERS 100
int main() {
    double radius = 5.0;
    double area = PI * radius * radius;
    printf("Max users allowed: %d\n", MAX_USERS);
    printf("Area of circle: %.2f\n", area);
    return 0;
}

2. Function-Like Macros

Macros can also take arguments, similar to functions. However, they do not have the overhead of a function call (like pushing variables to the stack). The preprocessor simply replaces the text inline.

#include <stdio.h>

// A macro to find the minimum of two numbers
#define MIN(a, b) ((a) < (b) ? (a) : (b))
int main() {
    int x = 10, y = 20;
    printf("The minimum is: %d\n", MIN(x, y));
    return 0;
}

3. Predefined Macros

ANSI C defines several standard macros that you can use anywhere in your code, which are extremely useful for debugging and logging.

• __FILE__: The current filename as a string.
• __LINE__: The current line number as an integer.
• __DATE__: The current date as a string.
• __TIME__: The current time as a string.