libc.info: iscntrl

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`iscntrl'--control character predicate

*Synopsis*
     #include <ctype.h>
     int iscntrl(int C);
   *Description*
`iscntrl' is a macro which classifies ASCII integer values by table
lookup.  It is a predicate returning non-zero for control characters,
and 0 for other characters.  It is defined only when `isascii'(C) is
true or C is EOF.
   You can use a compiled subroutine instead of the macro definition by
undefining the macro using ``#undef iscntrl''.
*Returns*
`iscntrl' returns non-zero if C is a delete character or ordinary
control character (`0x7F' or `0x00'-`0x1F').
*Portability*
`iscntrl' is ANSI C.
   No supporting OS subroutines are required.