Header Files in C

Header files are additional files in a C language containing definitions of different functions and their associated variables that need to be imported into a C program with the help of a preprocessor #include statement. All the header files have a ‘.h’ extension that contains C function declarations and macro definitions. The default header file that comes with the C compiler is the stdio.h.

Including a header file means using the content of the header file in your source program. A straightforward practice while programming in C or C++ programs is that you can keep every macro, global variables, constants, and other function prototypes in the header files.


#include <file>

Types of Header Files in C

There are 2 types of header files such as:-

Standard Library Header Files:– These are pre-existing header files which are available in C.
User-Defined Header Files:- The header files which are defined by the user then they are called user-defined header files. The #define directive is used to define a header file.

Different header files in C:-

You can use these standard C library functions by declaring header files. Different header files include different functions and different operations.
Below is the list of header files in C:-

Header FileWhat is does
stdio.hMainly used to perform input and output operations like print(), scanf().
string.hMainly used to perform string handling operations like strlen(), strcmp() etc.
conio.hWith this header file, you can execute console input and output operations.
stdlib.hMainly used to perform standard utility functions like malloc(), calloc() etc.
math.hMainly used to perform mathematical operations like sqrt(), pow() etc.
ctype.hUsed to perform character type functions like isdigit(), isalpha() etc.
time.hUsed to perform operations related to date and time.
errno.hFor performing error handling operations such as errno() to indicate the errors.


/ C program to understand the usage of header file.  
#include <stdio.h>  
#include <string.h>  
#include <math.h>  
int main()  
    char str1[10] = "hello";  
    char str2[10] = "Topperworld";  
    // function defined in math.h header file  
    long int a = pow(3, 3);   
    printf("The value of a is %d", a);  
    // function defined in string.h to calculate the length of the string  
    int length = strlen(str2);  
    printf("\nThe length of the string str2 is : %d", length);  
    return 0;  


The value of a is 27
The length of the string str2 is : 11

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