In C programming, a header file is a file that ends with the .h extension and contains features like functions, data types, macros, etc that can be used by any other C program by including that particular header file using "#include" preprocessor.
C language uses header files to provide the standard libraries and their components for use in programs.
Example:
C // Adding standard input and output library by // including stdio.h header file #include <stdio.h> int main() { // printf() function from stdio.h header file printf("Hello"); return 0; } In the above program, we have included the stdio.h header file that provides the standard input and output library in C.
Include Header Files
We have to include header files in our C program to use its features. There are two ways to do that:
C // for header files in system/default // directory #include <filename.h> // for Header files in same directory as // source file #include "filename.h"
The #include preprocessor directs the compiler that the header file needs to be processed before compilation and includes all the necessary data types and function definitions.
C Header FileTypes of C Header Files
There are two types of header files in C:
- Standard / Pre-existing header files
- Non-standard / User-defined header files
Standard Header Files in C
Standard header files contain the libraries defined in the ISO standard of the C programming language. They are stored in the system directory of the compiler and are present in all the C compilers from any vendor.
There are 31 standard header files in the latest version of C language. Following is the list of some commonly used header files in C:
Header File | Description |
|---|
| <assert.h> | It contains information for adding diagnostics that aid program debugging. |
| <errno.h> | It is used to perform error handling operations like errno(), strerror(), perror(), etc. |
| <float.h> | It contains a set of various platform-dependent constants related to floating point values. These constants are proposed by ANSI C. They make programs more portable. Some examples of constants included in this header file are- e(exponent), b(base/radix), etc. |
| <math.h> | It is used to perform mathematical operations like sqrt(), log2(), pow(), etc. |
| <signal.h> | It is used to perform signal handling functions like signal() and raise(). |
| <stdarg.h> | It is used to perform standard argument functions like va_start() and va_arg(). It is also used to indicate start of the variable-length argument list and to fetch the arguments from the variable-length argument list in the program respectively. |
| <ctype.h> | It contains function prototypes for functions that test characters for certain properties, and also function prototypes for functions that can be used to convert uppercase letters to lowercase letters and vice versa.
|
| <stdio.h> | It is used to perform input and output operations using functions like scanf(), printf(), etc. |
| <setjump.h> | It contains standard utility functions like malloc(), realloc(), etc. It contains function prototypes for functions that allow bypassing of the usual function call and return sequence. |
| <string.h> | It is used to perform various functionalities related to string manipulation like strlen(), strcmp(), strcpy(), size(), etc. |
| <limits.h> | It determines the various properties of the various variable types. The macros defined in this header limits the values of various variable types like char, int, and long. These limits specify that a variable cannot store any value beyond these limits, for example, an unsigned character can store up to a maximum value of 255. |
| <time.h> | It is used to perform functions related to date() and time() like setdate() and getdate(). It is also used to modify the system date and get the CPU time respectively. |
| <stddef.h> | It contains common type definitions used by C for performing calculations. |
| <locale.h> | It contains function prototypes and other information that enables a program to be modified for the current locale on which it’s running. It enables the computer system to handle different conventions for expressing data such as times, dates, or large numbers throughout the world. |
Example
The below example demonstrates the use of some commonly used header files in C.
C #include <math.h> #include <stdio.h> #include <stdlib.h> #include <string.h> int main(){ char s1[20] = "12345"; char s2[10] = "Geeks"; char s3[10] = "ForGeeks"; long int res; // Find the value of 9^3 using a // function in math.h library res = pow(9, 3); printf("Using math.h, " "The value is: %ld\n", res); // Convert a string to long long int // using a function in stdlib.h library long int a = atol(s1); printf("Using stdlib.h, the string"); printf(" to long int: %ld\n", a); // Copy the string s3 into s2 using // using a function in string.h library strcpy(s2, s3); printf("Using string.h, the strings" " s2 and s3: %s %s\n", s2, s3); return 0; } OutputUsing math.h, The value is: 729 Using stdlib.h, the string to long int: 12345 Using string.h, the strings s2 and s3: ForGeeks ForGeeks
Non-Standard Header Files
Non-standard header files are not part of the language's ISO standard. They are generally all the header files defined by the programmers for purposes like containing custom library functions etc or provided as external libraries by different vendors. They are manually installed by the user or maybe part of the compiler by some specific vendor.
There are lots of non-standard libraries for C language. Some commonly used non-standard/user-defined header files are listed below:
| Header File | Description |
|---|
| <conio.h> | It contains some useful console functions. |
| <gtk/gtk.h> | It contains GNU's GUI library for C. |
Example
The below example demonstrates the use of conio.h non-standard header file.
C #include<stdio.h> #include<conio.h> // Function to display a welcome message void displayMessage() { printf("Hello! Geek\n"); } int main() { // Using conio.h functions printf("Press any key to print message \n"); // Wait for a key press getch(); // Call the additional function after a key press displayMessage(); return 0; }
Output
Press any key to print message
Hello! Geek
Create your own Header File in C
Instead of writing a large and complex code again and again in different programs, we can create our own header files and include them in our program to use whenever we want. It enhances code functionality and readability. These header files are generally kept inside the source file directory and included using the second syntax to include header files, but we can also install them in the compilers default directory.
Refer to this article to know more - How Do I Create a Library in C?
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