what exactly is stdin?

[johnmerlino]

I know that the definition of stdin is the standard input stream where you can write data to. However, what kind of datatype is stdin? For example, below we put a character stored in second variable back into stdin. What data type and object is stdin?

ungetc(second, stdin);

[JLBorges]

#include <stdio.h>

extern FILE *stderr, *stdin, *stdout;

http://pubs.opengroup.org/onlinepubs/009695399/functions/stdin.html