Standard Functions :

The standard functions are built-in functions. In C programming language, the standard functions are declared in header files and defined in .dll files. In simple words, the standard functions can be defined as "the ready made functions defined by the system to make coding more easy". The standard functions are also called as library functions or pre-defined functions. 

In C when we use standard functions, we must include the respective header file using #include statement. For example, the function printf() is defined in header file stdio.h (Standard Input Output header file). When we use printf() in our program, we must include stdio.h header file using #include statement. 

C Programming Language provides the following header files with standard functions. 

Header File Purpose Example Functions 

stdio.h Provides functions to perform standard I/O operations printf(), scanf() 

conio.h Provides functions to perform console I/O operations clrscr(), getch() 

math.h Provides functions to perform mathematical operations sqrt(), pow() 

string.h Provides functions to handle string data values strlen(), strcpy() 

stdlib.h Provides functions to perform general functions calloc(), malloc()