when should i use #include<stdio.h>

Answer 1

stdio.h is the header file used for standard input and output operations. Without including this header file, you can't possibly display results on the output window.

edit: This's for C language.

Answer 2

This header is called <cstdio> instead of <stdio.h> in C++.
It can be useful, but please look at the <iostream> header if you're not familiar with it.

Answer 3

It tells the compiler to use functions, structures, macros and etc from file sdtio.h, which represents a part of glibc(or whatever is the standart C library you got). Compiler also adds record to the output executable "to-link list", that it should be linked to standart C library.

Answer 4

you shouldn‘t use it in c++ use #include <iostream> instead