Header Files in C

Header Files in C

A header file is a file with the extension .h that contains:

  • C function declarations

  • Macro definitions

  • Shared constants and variables

These are used to share code between multiple source files.

Types of Header Files

There are two types of header files:

  1. System Header Files

    • Provided with the compiler

    • Example: stdio.h

  2. User-Defined Header Files

    • Written by the programmer

    • Used to organize project-specific declarations

Including a Header File

You can include a header file using the C preprocessor directive:

#include

Including a header file is equivalent to copying its contents into your source file.
However, manually copying code is:

  • Error-prone

  • Hard to maintain

  • Not recommended, especially for multi-file programs

Common Practice

A standard practice in C is to store the following in header files:

  • Constants

  • Macros

  • System-wide global variables

  • Function prototypes

Then include that header file wherever needed.

Include Syntax

Both system and user header files use #include, but in different forms.

1. System Header Files

#include <file>

  • Searches for the file in standard system directories

  • You can add directories using the -I compiler option

2. User-Defined Header Files

#include "file"

  • Searches first in the current source file directory

  • Then in other include directories

  • You can add directories using the -I compiler option

Once-Only Headers (Header Guards)

If a header file is included more than once, the compiler may process it multiple times, causing errors.

To prevent this, use header guards:

#ifndef HEADER_FILE
#define HEADER_FILE

/* entire header file contents */

#endif

How It Works

  • The first time the file is included, HEADER_FILE is not defined, so the contents are included.

  • The macro is then defined.

  • If included again, the condition becomes false, and the file contents are skipped.

This technique is called a wrapper #ifndef or header guard.

Comments

Post a Comment

Popular posts from this blog

Programming in C GXEST204 - KTU 2024 scheme syllabus notes pdf ------- Dr Binu V P

About me About the course- Scheme and Evaluation GXEST204 Syllabus GXEST204 Model Question Paper GXEST204 Programming in C - KTU 2024 Scheme University Question Paper and Answer  Scheme GXEST204 Programming in C-May 2025  - KTU 2024 Scheme University Question Paper  and  Answer  Scheme  GXEST204 Programming in C-Dec 2025  - KTU 2024 Scheme Introduction to C Programming Beginning with C Programming Programming in C using gcc in Linux Debugging C Program using gdb Module-1 C Fundamentals Structure of a C Program Variables and constants in C Escape Sequences Keywords and Identifiers Data Types and qualifiers   Data Type summary Primitive and Derived Data Type Preprocessor Directive Header Files in C Input-output printf() and scanf() Operators, precedence and expressions Bitwise operators in C Simple Programs to Begin with Control Statements Control statements in C if-else statement , sample programs, programs to try switch case statement, ...
Read more

Structure of a C Program

Image
Basic Structure of a C Program A C program is divided into different sections. There are six main sections in a basic C program. The Six Sections Are: Documentation Section Include / Link / Preprocessor Section Definition Section Global Declarations Section (functions and variables) Main Function Subprogram Functions Overview Now let us learn about each of these sections in detail. Figure: Basic Structure Of C Program Sample Program The C program below finds the area of a circle using a user-defined function and a global variable PI holding the value of π. /* File Name: areaofcircle.c Author: Abhijith Date: 06/05/2021 Description: A program to calculate area of circle. User enters the radius. */ # include <stdio.h> // Link section # define PI 3.14 // Definition section float area ( float r); // Global declaration /****************************/ int main () // Main function { float r;...
Read more

Single and Multi Dimensional Arrays

Image
Arrays in C An array is a collection of data items, all of the same type , accessed using a common name . A one-dimensional array is like a list (vector) . A two-dimensional array is like a table (matrix) . Arrays can also have more dimensions . Memory Characteristics Array elements are stored in contiguous (adjacent) memory locations . Elements can be randomly accessed . The address of each element can be calculated using: Base address Size of each data element Declaring Single Dimensional Arrays Array variables are declared like normal variables, except: The variable name is followed by square brackets [ ] The brackets contain the array dimension (size) Rules Dimensions must be positive integers or constant expressions . In C99 , variables may be used as sizes if they hold a positive value at declaration. Memory is allocated only once when declared. Array size does not change later , even if the variable used changes. E...
Read more