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Library Functions in C Programming Language

Updated on July 9, 2018
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Raj is an ardent coder who loves exploring new technology. He is an IT pro with 9 years of exp in C#, Angular, React, Vue.

In this C language tutorial we will learn about library function available in C. Before I start explaining about library function let me refresh your idea about function. There are two type of function available in C, user defined functions, also know as UDF, and library functions.

A library function comes with C compiler setup program and we can directly use these function in our program by including corresponding header file. Library function contains many commonly used functions like mathematical function, string function, I/O function (to interact with display, keyboard etc). If you have written any program in C language then you must have used at least one library function. Commonly used library function is printf, scanf, clrscr, getch etc.

Whereas, UDF in C is function written by user (programmer) when there is no suitable library function available to fulfill his/her logical requirement. You can read more about User Defined Function (UDF) and its usage here.

Library function example


void main()
	printf("I am part of library function.");
Library function example
Library function example

C library function example with User Defined Function (UDF)


void greeting();

void main()

void greeting()
	printf("I am library function.\n");
	printf("But I am inside user defined function.");
Library function example
Library function example

In C, there are numerous library functions available to programmer. Here I am going to list some frequently used library function.

String functions in C (Requires “string.h”)

This header file contains all the functions related to string manipulation (or modification) like changing case of string to lower or upper, concatenating (or appending) one string to another etc. If you want to use any string library functions then you have to include “string.h”. Below is one simple example of few commonly used string library functions.

Example of string library program in C

#include "stdio.h"
#include "conio.h"
#include "string.h"

void main()
	char *name = "Raj Loves ";
	char *name2 = "C Program";
	int diff=0;


	printf("After comparison	:");
	diff = strcmp(name, name2);
	printf("%d\n", diff);

	strcat(name, name2);
	printf("After concatenating	:");

	printf("After Upper Case	:");

	printf("After Lower Case 	:");

	printf("After Reverse		:");

String functions example in C
String functions example in C

Explanation of string.h library function

As you can see this is a very simple example but, from my side, I think I should explain few lines of code and we will go line by line.

We have two character pointer variables (name and name2) to store some string. These two variables contains some string value (“Raj Loves ” and “C Program”).

Code Block 13 – 15 (strcmp in C): First we will compare both string and for that we will use library function “strcmp()”. The “strcmp()” function takes two character pointer parameter (click here to know more about parameter in function) and compare both strings. If parameter values are equal then this function returns 0 else returns their difference in from of integer (see line no. 14 for usage). Variable "diff" is used for storing the comparison result and as you can see in output window screen shot both string (“name” and “name2”) are not equal so this function returned their difference i.e. 15.

Code Block 17 – 19 (strcat in C): Next we have “strcat()” function which concatenates second string in first string. You might me thinking which one is first and second string. This method have following prototype “strcat(char *dest, char * src)” and we are passing “strcat(name, name2)”. That means variable “name” is destination pointer and “name2” is source pointer. So variable “name” will contain concatenated string i.e “Raj Loves C Program”.

Code Block 21 – 22 (strupr in C): In this block we are changing case of string available in “name” variable to upper case. As a result string “Raj Loves C Program” will become “RAJ LOVES C PROGRAM”. The “strupr()” function takes string (character pointer) as parameter and returns string (character pointer) in upper case.

Code Block 24 – 25 (strlwr in C): In this block we are changing case of string to lower case. As a result string “RAJ LOVES C PROGRAM” will become “raj loves c program”. The “strlwr()” function takes string (character pointer) as parameter and returns string (character pointer) in lower case.

Code Block 27 – 28 (strrev in C): In this block we are reversing string stored in “name” variable. As a result string “raj loves c program” will become “margorp c sevol jar”. The “strrev()” function takes string (character pointer) as parameter and returns string (character pointer) in lower case.

Apart from this, there is lots of other function available in “string.h”. Don’t hesitate to explore this header file.

Math functions in C (Requires “math.h”)

This header file contains all the functions required for mathematical calculation like sin, tan, log, sqrt, ceil, floor etc. To use any of the C math functions you have to include “math.h”. I am going to show you few C math library functions in below example.

Example of math library program in C

#include "stdio.h"
#include "conio.h"
#include "math.h"

void main()
	float num = 9.25;


	printf("Sqrt of %f	: ", num);
	printf("%f\n", sqrt(num));

	printf("Ceil of %f	: ", num);
	printf("%f\n", ceil(num));

	printf("Power of %f	: ", num);
	printf("%f\n", pow(num,num));

Math functions example in C
Math functions example in C

Explanation of math.h library function

I tried to keep this program as simple as I can. In this program, I am showing you uses of sqrt(), ceil() and pow() function of math library (or “math.h”).

Code Block 11 – 12 (sqrt in C): In this block we are calculating square root of value stored in “num” float variable, i.e. 9.25. Function used to calculate square root is very simple, “sqrt()” function takes one double type parameter and returns double type value. You can see the output in above screenshot.

Code Block 14 – 15 (ceil in C): In this block we are ceiling (rounding up) value stored in “num”. Function used for this purpose is “ceil()” which takes one double type parameter and returns double type value. Similarly if you want to floor (round down) any value then you can use “floor()” function to achieve that. The “floor()” function has same prototype as “ceil()” function.

Code Block 17 – 18 (pow in C): In this last important code block we are calculating power of value stored in “num” float variable, i.e. 9.25. Function used to calculate power is “pow()” function. Unlike other two function we used in this program, “pow()” takes two double type parameter and returns one double type value (obviously ;)). I have passed same “name” variable to “pow()” function and you can see the output in screenshot.

These are few functions from “math.h” header file. I suggest you to explore other math library function in your own program to see how those functions work.

Graphics function in C (requires “graphics.h”)

I find this library one of the very interesting libraries available in C programming language. When I used to teach C language that time I have written many small application using graphics in C. And obviously I will share those programs with you in my upcoming tutorials on graphics in C.

If want to draw line, circle, rectangle etc. in C then you have to use “graphics.h” in your program. I have one sample graphics program in C where it draws one line and one circle on screen. I know it’s a bit difficult to understand this program now (trust me this is the easiest graphic progam) and in this tutorial I am not going to explain you graphics concept. I properly commented code wherever I thought it is necessary. If you still find difficulty to understand this code, don’t worry we will have different session on graphics topic. Now smile and try to go through below code slowly, I am sure you will understand something from there.

Example of graphics program in C

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

int main()

	/* auto detect graphic driver */
	int gdriver = DETECT, gmode, errorcode;

	/* start graphics system */
	initgraph(&gdriver, &gmode, "G:\\TC\\BGI");

	/* check result if graphic initialization was successful */
	errorcode = graphresult();

	/* check error code for any error */
	if (errorcode != grOk)  
		printf("Error: %s\n", grapherrormsg(errorcode));
		printf("Press any key to exit:");
		exit(1);             /* return with error code */

	/* draw a line */
	line(0, 0, getmaxx()/3, getmaxy()/3);
	/* draw a circle */
	circle(getmaxx()/2, getmaxy()/2, 50);
	/* shut down C graphic system */
	return 0;

Graphics in C

graphics program in C
graphics program in C

Explanation of graphics.h library function

In this example of graphics library in C, we will concentrate on line no. 29 and 31. As function name is self explanatory, “line()” function draws line and “circle()” draws circle on screen. The “line()” function takes four parameter of integer type, first two parameter is for starting point X & Y coordinates and last two parameter is for ending point X & Y coordinates. Whereas “circle()” function takes three parameter of integer type, first two parameter is for X & Y coordinates and last parameter for radius of the circle.

Don’t you think these methods are really simple and easy to use? If you don’t think so then I would suggest you to read “Bresenham's line algorithm” and then compare that with “line()” function of graphics library of C.

Having a rich library in any programming language helps programmer to write less code and C language has no exception in that. Your C language library size will differ from vendor to vendor; few may have extra functions along with standard one.

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