getch() where is it getting from? k&r p.136

[vmars316]

Greetings,

int c, getch(void);
while (isspace(c = getch()))
;

getch() where is it getting a character from?
I checked The GNU Operating System but no getch (plenty of getchar).


Also where can I view a "C" programmers reference guide?

Thanks...vmars316