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Note POSIX compliance. Reword a little around the example.

.\"	$NetBSD: ctype.3,v 1.21 2010/05/06 11:46:11 jruoho Exp $
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.\"     @(#)ctype.3	6.5 (Berkeley) 4/19/91
.\"
.Dd May 6, 2010
.Dt CTYPE 3
.Os
.Sh NAME
.Nm isalpha ,
.Nm isupper ,
.Nm islower ,
.Nm isdigit ,
.Nm isxdigit ,
.Nm isalnum ,
.Nm isspace ,
.Nm ispunct ,
.Nm isprint ,
.Nm isgraph ,
.Nm iscntrl ,
.Nm isblank ,
.Nm toupper ,
.Nm tolower ,
.Nd character classification and mapping functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In ctype.h
.Fn isalpha "int c"
.Fn isupper "int c"
.Fn islower "int c"
.Fn isdigit "int c"
.Fn isxdigit "int c"
.Fn isalnum "int c"
.Fn isspace "int c"
.Fn ispunct "int c"
.Fn isprint "int c"
.Fn isgraph "int c"
.Fn iscntrl "int c"
.Fn isblank "int c"
.Fn toupper "int c"
.Fn tolower "int c"
.Sh DESCRIPTION
The above functions perform character tests and conversions on the integer
.Ar c .
.Pp
See the specific manual pages for information about the
test or conversion performed by each function.
.Sh EXAMPLES
To print an upper-case version of a string to stdout,
the following code can be used:
.Bd -literal -offset indent
const char *s = "xyz";

while (*s != '\e0') {
    putchar(toupper((int)(unsigned char)*s));
    s++;
}
.Ed
.Sh SEE ALSO
.Xr isalnum 3 ,
.Xr isalpha 3 ,
.Xr isblank 3 ,
.Xr iscntrl 3 ,
.Xr isdigit 3 ,
.Xr isgraph 3 ,
.Xr islower 3 ,
.Xr isprint 3 ,
.Xr ispunct 3 ,
.Xr isspace 3 ,
.Xr isupper 3 ,
.Xr isxdigit 3 ,
.Xr tolower 3 ,
.Xr toupper 3 ,
.Xr ascii 7
.Sh STANDARDS
These functions, with the exception of
.Fn isblank ,
conform to
.St -ansiC .
All described functions, including
.Fn isblank ,
also conform to
.St -p1003.1-2001 .
.Sh CAVEATS
The first argument of these functions is of type
.Vt int ,
but only a very restricted subset of values are actually valid.
The argument must either be the value of the macro
.Dv EOF
(which has a negative value),
or must be a non-negative value within the range representable as
.Vt unsigned char .
Passing invalid values leads to undefined behavior.
.Pp
Values of type
.Vt int
that were returned by
.Xr getc 3 ,
.Xr fgetc 3 ,
and similar functions or macros
are already in the correct range, and may be safely passed to these
.Nm ctype
functions without any casts.
.Pp
Values of type
.Vt char
or
.Vt signed char
must first be cast to
.Vt unsigned char ,
to ensure that the values are within the correct range.
The result should then be cast to
.Vt int
to avoid warnings from some compilers.
Casting a negative-valued
.Vt char
or
.Vt signed char
directly to
.Vt int
will produce a negative-valued
.Vt int ,
which will be outside the range of allowed values
(unless it happens to be equal to
.Dv EOF ,
but even that would not give the desired result).