C Standard Library

In this tutorial, you will learn about the C Standard Library. As C is one of the most popular and widely used languages among the programmers, it makes use of different libraries in the programs. The C Standard Library or simply C Library is a very exciting feature of C language is the set of inbuilt functions, constants and header files like <stdio.h>, <stdlib.h>, <math.h>, etc. C programmers will make use of these libraries as their reference manual.


You must include the header file in your program for accessing their functions. The header files will include the prototype and data definitions of their functions. For example, if you want to access the functions like printf()and scanf() then the header file ‘stdio.h’ must be included in your program. See the example below:


int main()
int x;
printf("Enter a number: ");
scanf("%d", &x);
printf("The numebr entered is: %d",x);

In the above program, the main() is the main function which must be present in every program for its execution. The main()function will be automatically called during the program execution. The scanf() and printf() functions fall under the header file ‘stdio.h’ so we have included ‘stdio.h’ before the main() function. The program will generate an error in the absence of the header file.

Why do we need library functions?

C language provides us different header files which will help us 0in writing good and efficient programs. Following reasons will help to clarify more on why header files are needed.

  • They are reliable and are easy to use in our program.
  • These library functions are easily portable.
  • They are used for creating the most efficient and optimized code.
  • As they can be used easily anytime from anywhere to it will help to save the time of the programmers in the development of the applications.

See the below example to see the implementation of the header file ‘math.h’ for computing the length of the given square with the given area.

C program to compute the length of the square when the area is given

The list of C Standard Library header files

Header FilesMeaning
<assert.h>used for asserting programs
<ctype.h>includes Character type functions
<locale.h>includes Localization functions
<math.h>includes Mathematics functions
<setjmp.h>includes Jump functions
<signal.h>includes signal handling functions
<stdarg.h>Variable arguments handling functions
<stdio.h>inlcudes all standard Input/Output operations.
<stdlib.h>Implements all Standard Utility functions
<string.h>includes all String handling functions
<time.h>includes date time function




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