#include <ctype.h>
int iscntrl(int c);
iscntrl returns nonzero (true) if and only if the value of the argument c is a control character. Control characters have values 0 through 31 and the single value 127.
iscntrl conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).