As you might have noticed, we included some libraries throughout the examples such as the stdio.h. In the following, we will show how to define own header files and include them into a C program.

Standard Headers

CAUTION: When using non-standard headers, always keep the header files within your working directory and include the header via "header.h" instead of <header.h>. In fact, you can use "header.h" even if using standard headers, but that's not recommended.

There are some standad headers provided by the standard C libraries. Some of those are listed in the following table:

Header File Use
assert.h error searching and debugging
ctype.h char testing and converting
errno.h error codes
float.h Limits/Properties for floating points
limits.h Constants
locale.h country-specific properties
math.h mathematical functions
setjmp.h unconditional jumps
signal.h Signals
stdarg.h variable parameter delivery
stddef.h Standard data types
stdio.h Standard-I/O
stdlib.h useful functions
string.h String operations
time.h Date and Time
complex.h complex arithmetic (trigonometry etc.)
Fenv.h control of floating point environment
inttypes.h for precise integer types
iso646.h alternative notation for logical operations (in the ISO-646 format)
stdbool.h boolean data types
stdint.h integer types with a preset width
tgmath.h type-generic mathematical functions
wchar.h wide chars
wctype.h ctype for wide chars

Header File

As there are standard headers to include, it is also possible defining own headers. This is especailly useful for codes that one mighty use more often as it spares declaring a function all over again and it also makes sure that a function can be fixed at one place only if there are errors. In the following, a sample header file will be shown.


#ifndef MATHCOLLECTION_H_   /* Include guard */

// code to include in the header
int mod(int a, int b);  /* function declaration */
int gcd(int a, int b);
int power(int num, int exp);


NOTE: The keyword #ifndef makes sure to only define this header if the token is not included already. If included already, the #else block will be used. This example does not utilize any #else block.

Function Definitions


#include "mathcollection.h"     // Not neccessary here

int mod(int a, int b){
    while(a >= b)
        a = a - b;
    return a;

int gcd(int a, int b){
    if (b == 0)
        return a;
    a = mod(a, b);
    return gcd(b, a);

int power(int num, int exp){
    int res = 1;
        res *= num;
    return num;

Main Program

The following C program will include the previously created header file and use the defined functions. The expected results are shown in the comments next to the respective lines.


#include "mathcollection.h"

int main(){
    int a = 16;
    int b = 3;

    int mod_res = mod(a, b);    // mod(16, 3) = 1 as 15 % 3 = 0
    int gcd_res = gcd(a, b);    // gcd(16, 3) = 1 as 3 is prime
    int pow_res = power(a, b); // 16*16*16

    return 0;