Header files and include directive - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a header file in C?

A header file in C is a file with extension .h that contains declarations of functions, macros, and variables. It helps share code between different source files.

beginner

What does the #include directive do?

The #include directive tells the compiler to copy the contents of a specified header file into the source file before compilation. It helps reuse code and declarations.

intermediate

Difference between #include <file.h> and #include "file.h"?

#include <file.h> searches for the header file in system directories.<br>#include "file.h" searches first in the current directory, then system directories.

intermediate

Why use header guards in header files?

Header guards prevent a header file from being included multiple times in the same source file, avoiding errors like redefinition of functions or variables.

beginner

Example of a simple header guard in a header file?

#ifndef MYHEADER_H #define MYHEADER_H // Declarations here #endif This code ensures the header content is included only once.

What is the main purpose of a header file in C?

A To store function definitions

B To execute code

C To declare functions and macros for sharing

D To compile the program

Header files declare functions and macros so multiple source files can use them.

Which directive includes a header file from the system directories?

A #include <file.h>

B #define file.h

C #import <file.h>

D #include "file.h"

#include <file.h> searches system directories for the header file.

What problem do header guards solve?

A Allow dynamic linking

B Speed up compilation

C Make code run faster

D Prevent multiple inclusions causing errors

Header guards stop the same header file from being included more than once.

Where does the compiler copy the contents of a header file?

A Before compilation

B During linking

C At runtime

D After compilation

The preprocessor copies header file contents into the source file before compilation.

Which is the correct way to write a header guard?

A #include HEADER_H

B #ifndef HEADER_H #define HEADER_H #endif

C #define HEADER_H #ifndef HEADER_H

D #ifdef HEADER_H #define HEADER_H

The pattern #ifndef ... #define ... #endif is used for header guards.

Explain what a header file is and why we use the #include directive in C.

Think about how multiple files share code.

Describe how header guards work and why they are important.

Imagine including the same file twice by mistake.