Unit 3 · Lesson 124 minAcademic review pending

Code before the compiler: the preprocessor

Use headers, symbolic macros, include guards, and conditional compilation without hiding program logic.

Choose explanation

After this lesson

You should be able to

Explain how preprocessing changes a translation unit before compilation.
Use object-like macros and include guards safely.

Text transformation with a narrow job

Preprocessor directives begin with # and run before normal C compilation. #include brings declarations from a header into the translation unit. Angle brackets search implementation header paths; quotes normally search the project location first.

#define performs token substitution; it does not create a typed variable. Prefer const objects or enum constants when type and scope matter. Use macros for genuine preprocessing needs, and parenthesize every parameter and the full result of a function-like macro.

A guarded header

#ifndef SCORE_H
#define SCORE_H

int clamp_score(int score);

#endif

Compile one of several versions

#if evaluates a preprocessing constant expression. #ifdef checks whether a name is defined; #ifndef checks whether it is absent. These directives can select platform-specific code or debugging diagnostics before compilation.

#undef removes a macro definition from that point onward. Conditional compilation should remain small and explicit; scattering feature branches through business logic makes every version difficult to reason about.

Optional diagnostic

#ifdef DEBUG
fprintf(stderr, "score=%d\n", score);
#endif

Try it yourself

Write an include guard for a header named lesson.h.

Need a hint?

Use a project-specific uppercase name and pair #ifndef with #define and #endif.

Check the worked solution

A guard prevents the header body from being processed more than once in one translation unit.

#ifndef CHADUVU_BIDDA_LESSON_H
#define CHADUVU_BIDDA_LESSON_H

/* declarations */

#endif

Quick check

Which directive tests that a macro name has not been defined?

Select an answer to check your thinking.

Why this lesson exists

Syllabus mapping

include · define · undef · if · ifdef · ifndef directives

Maps to course outcomes CO2, CO3, CO4.