The iscntrl() function shall test whether c is a character of
class cntrl in the program's current locale; see the Base Definitions
volume of IEEE Std 1003.1-2001, Chapter 7, Locale.
The c argument is a type int, the value of which the application
shall ensure is a character representable as an unsigned char or equal
to the value of the macro EOF. If the argument has any other value, the
behavior is undefined.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html