C Header Files

C header files are files containing declarations and definitions of functions, variables, and macros that can be used in multiple C source files. They help with code organization, code reuse, and separation of interface and implementation. Header files typically have a .h extension and are included in C source files using the #include preprocessor directive.

Types of header files

There are two types of header files: system header files and user-defined header files. System header files are provided by the compiler and contain declarations for functions and variables that are part of the standard C library. User-defined header files are created by the programmer and can contain declarations for functions and variables that are specific to the program.

Here are some examples of system header files:

  1. stdio.h: Standard input/output functions
  2. stdlib.h: Standard library functions, such as malloc() and free()
  3. math.h: Mathematical functions, such as sin() and cos()
  4. string.h: String manipulation functions, such as strcpy() and strcat()

Here are some examples of user-defined header files:

  1. my_functions.h: Declarations for my own functions
  2. my_types.h: Declarations for my own types
  3. my_constants.h: Declarations for my own constants

Declaration vs. Definition

Header files primarily contain declarations rather than definitions. Declarations tell the compiler about the existence and data types of functions, variables, and macros, while definitions provide the actual implementation. Definitions are typically placed in source files (.c files).

Include Guard

To prevent multiple inclusions of the same header file, an include guard is often used. This ensures that the header file is included only once in each source file.

#ifndef MYHEADER_H #define MYHEADER_H // Declarations and definitions go here #endif

This protects against multiple inclusions of the same header file in a single source file.

Standard Library Headers

C provides a set of standard library headers that are included using the #include directive.

#include <stdio.h> #include <stdlib.h>

These headers provide declarations for functions like printf, scanf, malloc, and more.

Custom Header Files

Custom header files are created to organize code and provide a clean separation between interface and implementation. For example, if you have a set of utility functions, you can create a header file and source file like this:

myutils.h (Header file):
#ifndef MYUTILS_H #define MYUTILS_H int add(int a, int b); int subtract(int a, int b); #endif
myutils.c (Source file):
#include "myutils.h" int add(int a, int b) { return a + b; } int subtract(int a, int b) { return a - b; }

Other C source files can include myutils.h to access the add and subtract functions without needing to know their implementation details.

Header File Inclusion

To use the declarations and definitions from a header file in your C source code, you include the header file using #include.

#include "myutils.h" int main() { int result = add(5, 3); printf("Result: %d\n", result); return 0; }

In this example, we include myutils.h to access the add function.

Compiler Search Paths

When including custom header files using double quotes (e.g., #include "myutils.h"), the compiler searches in the current directory for the header file. When using angle brackets (e.g., #include <stdio.h> , the compiler searches in standard system directories.


Header files are essential for building modular and maintainable C code. They allow you to separate the interface from the implementation, promote code reuse, and provide clear documentation for functions, variables, and macros that are shared among different parts of your program.