<s>
C	B-Language
character	I-Language
classification	I-Language
is	O
an	O
operation	O
provided	O
by	O
a	O
group	O
of	O
functions	O
in	O
the	O
ANSI	B-Language
C	I-Language
Standard	I-Language
Library	I-Language
for	O
the	O
C	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Early	O
C-language	O
programmers	O
working	O
on	O
the	O
Unix	B-Application
operating	I-Application
system	I-Application
developed	O
programming	O
idioms	O
for	O
classifying	O
characters	O
into	O
different	O
types	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
the	O
ASCII	B-Protocol
character	I-Protocol
set	I-Protocol
,	O
the	O
following	O
expression	O
identifies	O
a	O
letter	O
,	O
when	O
its	O
value	O
is	O
true	O
:	O
</s>
<s>
As	O
this	O
may	O
be	O
expressed	O
in	O
multiple	O
formulations	O
,	O
it	O
became	O
desirable	O
to	O
introduce	O
short	O
,	O
standardized	O
forms	O
of	O
such	O
tests	O
that	O
were	O
placed	O
in	O
the	O
system-wide	O
header	O
file	O
ctype.h.	O
</s>
<s>
In	O
most	O
C	B-Language
libraries	I-Language
,	O
they	O
are	O
written	O
as	O
static	O
table	O
lookups	O
instead	O
of	O
macros	O
or	O
functions	O
.	O
</s>
<s>
Early	O
versions	O
of	O
Linux	B-Application
used	O
a	O
potentially	O
faulty	O
method	O
similar	O
to	O
the	O
first	O
code	O
sample	O
:	O
</s>
<s>
The	O
functions	O
that	O
operate	O
on	O
single-byte	O
characters	O
are	O
defined	O
in	O
ctype.h	B-Language
header	O
file	O
(	O
cctype	B-Language
in	O
C++	O
)	O
.	O
</s>
<s>
The	O
functions	O
that	O
operate	O
on	O
wide	O
characters	O
are	O
defined	O
in	O
wctype.h	B-Language
header	O
file	O
(	O
cwctype	B-Language
in	O
C++	O
)	O
.	O
</s>
