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Library or Built-in functions In C Language
• Library functions in C language are inbuilt functions which are grouped together and placed in a common place called library.
• Each library function in C performs specific operation.
• We can make use of these library functions to get the pre-defined output instead of writing our own code to get those outputs.
• These library functions are created by the persons who designed and created C compilers.
• All C standard library functions are declared in many header files which are saved as filename.h
• We are including these header files in our C program using “#includes<filename.h>” command to make use of the functions those are declared in the header files.
• When we include header files in our C program using “#include<filename.h>” command, all C code of the header files are included in C program. Then, this C program is compiled by compiler and executed.

List of most used header files in C :

Check the below table to know all the C library functions and header files in which they are declared.
S.No Header file Description
1 stdio.h This is standard input/output header file in which Input/Output functions are declared
2 conio.h This is console input/output header file
3 string.h All string related functions are defined in this header file
4 stdlib.h This header file contains general functions used in C programs
5 math.h All maths related functions are defined in this header file
6 time.h This header file contains time and clock related functions
7 ctype.h All character handling functions are defined in this header file
8 stdarg.h Variable argument functions are declared in this header file
9 signal.h Signal handling functions are declared in this file
10 setjmp.h This file contains all jump functions
11 locale.h This file contains locale functions
12 errno.h Error handling functions are given in this file
13 assert.h This contains diagnostics functions