Macros and the Preprocessor

The preprocessor handles directives beginning with # before compilation. It can define symbols, keep code conditionally, and include header contents. This is a textual preparation stage, so it does not understand variable types.

Macros

A macro replaces a name with text. Macros used as constants conventionally use uppercase names.

#include <stdio.h>

#define PI 3.1415926

double circle_area(double radius) {
    return PI * radius * radius;
}

int main(void) {
    double radius = 3.0;
    printf("area = %.2f\n", circle_area(radius));
    return 0;
}

Function-like macros

A function-like macro resembles a function call, but creates no stack frame and performs no type checking. Arguments may be full expressions, so parenthesize every parameter use and the complete result. Avoid side-effecting arguments such as i++ when a macro repeats its parameter.

#include <stdio.h>

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

int main(void) {
    int n = SQUARE(3 + 1);
    printf("n = %d\n", n);
    return 0;
}

Conditional compilation

#if selects source code when its condition is true, #else provides the alternative, and #endif closes the conditional group. These directives run before compilation, unlike a runtime if/else.

This circle-area calculator has two build modes: a debug build shows the radius and calculation result, while a release build shows only the area the user needs. Change DEBUG and recompile to switch the output.

#include <stdio.h>

#define PI 3.1415926
#define DEBUG 1

int main(void) {
    double radius = 3.0;
    double area = PI * radius * radius;

#if DEBUG
    printf("[DEBUG] radius = %.2f, area = %.2f\n", radius, area);
#else
    printf("Area: %.2f\n", area);
#endif

    return 0;
}

With DEBUG set to 1, the preprocessor keeps the #if branch and the output is [DEBUG] radius = 3.00, area = 28.27. Set it to 0 and recompile to keep only the #else branch, producing Area: 28.27. The unselected branch is not compiled.

#ifdef and #ifndef check whether a macro is defined; the next section uses #ifndef to protect a header.