fflush(stdin) ANSI C
c, fflush
Solution
You must have `#include <stdio.h>` when you call any function declared in that header.
Ok, that's not quite true. In the 1989/1990 version of the language standard, a call can create an implicit declaration of a function. If that happens to match the correct declaration, you can get away with it;. Otherwise your program's behavior is undefined -- which means you still might get away with it, or not, but the compiler isn't required to warn you about it. Since `printf` takes a variable number of arguments, you must have a visible declaration to avoid undefined behavior -- and the way to get a visible declaration is `#include <stdio.h>`.
(You can also declare the function yourself, but that's error-prone, and there's no good reason to do that.)
In C99 and later, there are no implicit declarations.
`main()` should be `int main(void)`.
`fflush(stdin)` has undefined behavior. If you want to discard characters entered after the `scanf()` call, you can read and discard them.
Problem
I am a beginner in ANSI C, and I have a question, it may be silly question and I am sorry for it. ``` #include<stdio.h> main() { int age; printf("Hello World!\n"); printf("Please enter your age: "); scanf("%d", &age); printf("You entered %d\n", age); fflush(stdin); getchar(); } ``` It is my second program to learn scanf function. My question is : I know that printf, scanf, fflush, stdin, and getchar are defined in stdio.h but only when I use fflush(stdin) I must put #include<stdio.h>, but when use any other method I can remove that line #include.