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<a name="Character-Handling"></a>
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<a name="Character-Handling-1"></a>
<h1 class="chapter">4. Character Handling</h1>

<p>Programs that work with characters and strings often need to classify a
character&mdash;is it alphabetic, is it a digit, is it whitespace, and so
on&mdash;and perform case conversion operations on characters.  The
functions in the header file &lsquo;<tt>ctype.h</tt>&rsquo; are provided for this
purpose.
<a name="index-ctype_002eh"></a>
</p>
<p>Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale.  (More precisely, they are affected
by the locale currently selected for character classification&mdash;the
<code>LC_CTYPE</code> category; see <a href="libc_7.html#Locale-Categories">Categories of Activities that Locales Affect</a>.)
</p>
<p>The ISO C standard specifies two different sets of functions.  The
one set works on <code>char</code> type characters, the other one on
<code>wchar_t</code> wide characters (see section <a href="libc_6.html#Extended-Char-Intro">Introduction to Extended Characters</a>).
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Classification-of-Characters">4.1 Classification of Characters</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">       Testing whether characters are
			                letters, digits, punctuation, etc.
</td></tr>
<tr><td align="left" valign="top"><a href="#Case-Conversion">4.2 Case Conversion</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                    Case mapping, and the like.
</td></tr>
<tr><td align="left" valign="top"><a href="#Classification-of-Wide-Characters">4.3 Character class determination for wide characters</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top"></td></tr>
<tr><td align="left" valign="top"><a href="#Using-Wide-Char-Classes">4.4 Notes on using the wide character classes</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top"></td></tr>
<tr><td align="left" valign="top"><a href="#Wide-Character-Case-Conversion">4.5 Mapping of wide characters.</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top"></td></tr>
</table>

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<a name="Classification-of-Characters-1"></a>
<h2 class="section">4.1 Classification of Characters</h2>
<a name="index-character-testing"></a>
<a name="index-classification-of-characters"></a>
<a name="index-predicates-on-characters"></a>
<a name="index-character-predicates"></a>

<p>This section explains the library functions for classifying characters.
For example, <code>isalpha</code> is the function to test for an alphabetic
character.  It takes one argument, the character to test, and returns a
nonzero integer if the character is alphabetic, and zero otherwise.  You
would use it like this:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">if (isalpha (c))
  printf (&quot;The character `%c' is alphabetic.\n&quot;, c);
</pre></td></tr></table>

<p>Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with &lsquo;<samp>is</samp>&rsquo;.
Each of them takes one argument, which is a character to test, and
returns an <code>int</code> which is treated as a boolean value.  The
character argument is passed as an <code>int</code>, and it may be the
constant value <code>EOF</code> instead of a real character.
</p>
<p>The attributes of any given character can vary between locales.
See section <a href="libc_7.html#Locales">Locales and Internationalization</a>, for more information on locales.
</p>
<p>These functions are declared in the header file &lsquo;<tt>ctype.h</tt>&rsquo;.
<a name="index-ctype_002eh-1"></a>
</p>
<a name="index-lower_002dcase-character"></a>
<dl>
<dt><a name="index-islower"></a><u>Function:</u> int <b>islower</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a lower-case letter.  The letter need not be
from the Latin alphabet, any alphabet representable is valid.
</p></dd></dl>

<a name="index-upper_002dcase-character"></a>
<dl>
<dt><a name="index-isupper"></a><u>Function:</u> int <b>isupper</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is an upper-case letter.  The letter need not be
from the Latin alphabet, any alphabet representable is valid.
</p></dd></dl>

<a name="index-alphabetic-character"></a>
<dl>
<dt><a name="index-isalpha"></a><u>Function:</u> int <b>isalpha</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is an alphabetic character (a letter).  If
<code>islower</code> or <code>isupper</code> is true of a character, then
<code>isalpha</code> is also true.
</p>
<p>In some locales, there may be additional characters for which
<code>isalpha</code> is true&mdash;letters which are neither upper case nor lower
case.  But in the standard <code>&quot;C&quot;</code> locale, there are no such
additional characters.
</p></dd></dl>

<a name="index-digit-character"></a>
<a name="index-decimal-digit-character"></a>
<dl>
<dt><a name="index-isdigit"></a><u>Function:</u> int <b>isdigit</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a decimal digit (&lsquo;<samp>0</samp>&rsquo; through &lsquo;<samp>9</samp>&rsquo;).
</p></dd></dl>

<a name="index-alphanumeric-character"></a>
<dl>
<dt><a name="index-isalnum"></a><u>Function:</u> int <b>isalnum</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is an alphanumeric character (a letter or
number); in other words, if either <code>isalpha</code> or <code>isdigit</code> is
true of a character, then <code>isalnum</code> is also true.
</p></dd></dl>

<a name="index-hexadecimal-digit-character"></a>
<dl>
<dt><a name="index-isxdigit"></a><u>Function:</u> int <b>isxdigit</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a hexadecimal digit.
Hexadecimal digits include the normal decimal digits &lsquo;<samp>0</samp>&rsquo; through
&lsquo;<samp>9</samp>&rsquo; and the letters &lsquo;<samp>A</samp>&rsquo; through &lsquo;<samp>F</samp>&rsquo; and
&lsquo;<samp>a</samp>&rsquo; through &lsquo;<samp>f</samp>&rsquo;.
</p></dd></dl>

<a name="index-punctuation-character"></a>
<dl>
<dt><a name="index-ispunct"></a><u>Function:</u> int <b>ispunct</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a punctuation character.
This means any printing character that is not alphanumeric or a space
character.
</p></dd></dl>

<a name="index-whitespace-character"></a>
<dl>
<dt><a name="index-isspace"></a><u>Function:</u> int <b>isspace</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a <em>whitespace</em> character.  In the standard
<code>&quot;C&quot;</code> locale, <code>isspace</code> returns true for only the standard
whitespace characters:
</p>
<dl compact="compact">
<dt> <code>' '</code></dt>
<dd><p>space
</p>
</dd>
<dt> <code>'\f'</code></dt>
<dd><p>formfeed
</p>
</dd>
<dt> <code>'\n'</code></dt>
<dd><p>newline
</p>
</dd>
<dt> <code>'\r'</code></dt>
<dd><p>carriage return
</p>
</dd>
<dt> <code>'\t'</code></dt>
<dd><p>horizontal tab
</p>
</dd>
<dt> <code>'\v'</code></dt>
<dd><p>vertical tab
</p></dd>
</dl>
</dd></dl>

<a name="index-blank-character"></a>
<dl>
<dt><a name="index-isblank"></a><u>Function:</u> int <b>isblank</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a blank character; that is, a space or a tab.
This function was originally a GNU extension, but was added in ISO C99.
</p></dd></dl>

<a name="index-graphic-character"></a>
<dl>
<dt><a name="index-isgraph"></a><u>Function:</u> int <b>isgraph</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a graphic character; that is, a character
that has a glyph associated with it.  The whitespace characters are not
considered graphic.
</p></dd></dl>

<a name="index-printing-character"></a>
<dl>
<dt><a name="index-isprint"></a><u>Function:</u> int <b>isprint</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a printing character.  Printing characters
include all the graphic characters, plus the space (&lsquo;<samp> </samp>&rsquo;) character.
</p></dd></dl>

<a name="index-control-character"></a>
<dl>
<dt><a name="index-iscntrl"></a><u>Function:</u> int <b>iscntrl</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a control character (that is, a character that
is not a printing character).
</p></dd></dl>

<a name="index-ASCII-character"></a>
<dl>
<dt><a name="index-isascii"></a><u>Function:</u> int <b>isascii</b><i> (int <var>c</var>)</i></dt>
<dd><p>Returns true if <var>c</var> is a 7-bit <code>unsigned char</code> value that fits
into the US/UK ASCII character set.  This function is a BSD extension
and is also an SVID extension.
</p></dd></dl>

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<a name="Case-Conversion-1"></a>
<h2 class="section">4.2 Case Conversion</h2>
<a name="index-character-case-conversion"></a>
<a name="index-case-conversion-of-characters"></a>
<a name="index-converting-case-of-characters"></a>

<p>This section explains the library functions for performing conversions
such as case mappings on characters.  For example, <code>toupper</code>
converts any character to upper case if possible.  If the character
can&rsquo;t be converted, <code>toupper</code> returns it unchanged.
</p>
<p>These functions take one argument of type <code>int</code>, which is the
character to convert, and return the converted character as an
<code>int</code>.  If the conversion is not applicable to the argument given,
the argument is returned unchanged.
</p>
<p><strong>Compatibility Note:</strong> In pre-ISO C dialects, instead of
returning the argument unchanged, these functions may fail when the
argument is not suitable for the conversion.  Thus for portability, you
may need to write <code>islower(c) ? toupper(c) : c</code> rather than just
<code>toupper(c)</code>.
</p>
<p>These functions are declared in the header file &lsquo;<tt>ctype.h</tt>&rsquo;.
<a name="index-ctype_002eh-2"></a>
</p>
<dl>
<dt><a name="index-tolower"></a><u>Function:</u> int <b>tolower</b><i> (int <var>c</var>)</i></dt>
<dd><p>If <var>c</var> is an upper-case letter, <code>tolower</code> returns the corresponding
lower-case letter.  If <var>c</var> is not an upper-case letter,
<var>c</var> is returned unchanged.
</p></dd></dl>

<dl>
<dt><a name="index-toupper"></a><u>Function:</u> int <b>toupper</b><i> (int <var>c</var>)</i></dt>
<dd><p>If <var>c</var> is a lower-case letter, <code>toupper</code> returns the corresponding
upper-case letter.  Otherwise <var>c</var> is returned unchanged.
</p></dd></dl>

<dl>
<dt><a name="index-toascii"></a><u>Function:</u> int <b>toascii</b><i> (int <var>c</var>)</i></dt>
<dd><p>This function converts <var>c</var> to a 7-bit <code>unsigned char</code> value
that fits into the US/UK ASCII character set, by clearing the high-order
bits.  This function is a BSD extension and is also an SVID extension.
</p></dd></dl>

<dl>
<dt><a name="index-_005ftolower"></a><u>Function:</u> int <b>_tolower</b><i> (int <var>c</var>)</i></dt>
<dd><p>This is identical to <code>tolower</code>, and is provided for compatibility
with the SVID.  See section <a href="libc_1.html#SVID">SVID (The System V Interface Description)</a>.
</p></dd></dl>

<dl>
<dt><a name="index-_005ftoupper"></a><u>Function:</u> int <b>_toupper</b><i> (int <var>c</var>)</i></dt>
<dd><p>This is identical to <code>toupper</code>, and is provided for compatibility
with the SVID.
</p></dd></dl>


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<a name="Character-class-determination-for-wide-characters"></a>
<h2 class="section">4.3 Character class determination for wide characters</h2>

<p>Amendment 1 to ISO C90 defines functions to classify wide
characters.  Although the original ISO C90 standard already defined
the type <code>wchar_t</code>, no functions operating on them were defined.
</p>
<p>The general design of the classification functions for wide characters
is more general.  It allows extensions to the set of available
classifications, beyond those which are always available.  The POSIX
standard specifies how extensions can be made, and this is already
implemented in the GNU C library implementation of the <code>localedef</code>
program.
</p>
<p>The character class functions are normally implemented with bitsets,
with a bitset per character.  For a given character, the appropriate
bitset is read from a table and a test is performed as to whether a
certain bit is set.  Which bit is tested for is determined by the
class.
</p>
<p>For the wide character classification functions this is made visible.
There is a type classification type defined, a function to retrieve this
value for a given class, and a function to test whether a given
character is in this class, using the classification value.  On top of
this the normal character classification functions as used for
<code>char</code> objects can be defined.
</p>
<dl>
<dt><a name="index-wctype_005ft"></a><u>Data type:</u> <b>wctype_t</b></dt>
<dd><p>The <code>wctype_t</code> can hold a value which represents a character class.
The only defined way to generate such a value is by using the
<code>wctype</code> function.
</p>
<a name="index-wctype_002eh"></a>
<p>This type is defined in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-wctype"></a><u>Function:</u> wctype_t <b>wctype</b><i> (const char *<var>property</var>)</i></dt>
<dd><p>The <code>wctype</code> returns a value representing a class of wide
characters which is identified by the string <var>property</var>.  Beside
some standard properties each locale can define its own ones.  In case
no property with the given name is known for the current locale
selected for the <code>LC_CTYPE</code> category, the function returns zero.
</p>
<p>The properties known in every locale are:
</p>
<table>
<tr><td width="25%"><code>&quot;alnum&quot;</code></td><td width="25%"><code>&quot;alpha&quot;</code></td><td width="25%"><code>&quot;cntrl&quot;</code></td><td width="25%"><code>&quot;digit&quot;</code></td></tr>
<tr><td width="25%"><code>&quot;graph&quot;</code></td><td width="25%"><code>&quot;lower&quot;</code></td><td width="25%"><code>&quot;print&quot;</code></td><td width="25%"><code>&quot;punct&quot;</code></td></tr>
<tr><td width="25%"><code>&quot;space&quot;</code></td><td width="25%"><code>&quot;upper&quot;</code></td><td width="25%"><code>&quot;xdigit&quot;</code></td></tr>
</table>

<a name="index-wctype_002eh-1"></a>
<p>This function is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<p>To test the membership of a character to one of the non-standard classes
the ISO C standard defines a completely new function.
</p>
<dl>
<dt><a name="index-iswctype"></a><u>Function:</u> int <b>iswctype</b><i> (wint_t <var>wc</var>, wctype_t <var>desc</var>)</i></dt>
<dd><p>This function returns a nonzero value if <var>wc</var> is in the character
class specified by <var>desc</var>.  <var>desc</var> must previously be returned
by a successful call to <code>wctype</code>.
</p>
<a name="index-wctype_002eh-2"></a>
<p>This function is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<p>To make it easier to use the commonly-used classification functions,
they are defined in the C library.  There is no need to use
<code>wctype</code> if the property string is one of the known character
classes.  In some situations it is desirable to construct the property
strings, and then it is important that <code>wctype</code> can also handle the
standard classes.
</p>
<a name="index-alphanumeric-character-1"></a>
<dl>
<dt><a name="index-iswalnum"></a><u>Function:</u> int <b>iswalnum</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>This function returns a nonzero value if <var>wc</var> is an alphanumeric
character (a letter or number); in other words, if either <code>iswalpha</code>
or <code>iswdigit</code> is true of a character, then <code>iswalnum</code> is also
true.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;alnum&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-3"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-alphabetic-character-1"></a>
<dl>
<dt><a name="index-iswalpha"></a><u>Function:</u> int <b>iswalpha</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is an alphabetic character (a letter).  If
<code>iswlower</code> or <code>iswupper</code> is true of a character, then
<code>iswalpha</code> is also true.
</p>
<p>In some locales, there may be additional characters for which
<code>iswalpha</code> is true&mdash;letters which are neither upper case nor lower
case.  But in the standard <code>&quot;C&quot;</code> locale, there are no such
additional characters.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;alpha&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-4"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-control-character-1"></a>
<dl>
<dt><a name="index-iswcntrl"></a><u>Function:</u> int <b>iswcntrl</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a control character (that is, a character that
is not a printing character).
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;cntrl&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-5"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-digit-character-1"></a>
<dl>
<dt><a name="index-iswdigit"></a><u>Function:</u> int <b>iswdigit</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a digit (e.g., &lsquo;<samp>0</samp>&rsquo; through &lsquo;<samp>9</samp>&rsquo;).
Please note that this function does not only return a nonzero value for
<em>decimal</em> digits, but for all kinds of digits.  A consequence is
that code like the following will <strong>not</strong> work unconditionally for
wide characters:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">n = 0;
while (iswdigit (*wc))
  {
    n *= 10;
    n += *wc++ - L'0';
  }
</pre></td></tr></table>

<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;digit&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-6"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-graphic-character-1"></a>
<dl>
<dt><a name="index-iswgraph"></a><u>Function:</u> int <b>iswgraph</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a graphic character; that is, a character
that has a glyph associated with it.  The whitespace characters are not
considered graphic.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;graph&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-7"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-lower_002dcase-character-1"></a>
<dl>
<dt><a name="index-iswlower"></a><u>Function:</u> int <b>iswlower</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a lower-case letter.  The letter need not be
from the Latin alphabet, any alphabet representable is valid.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;lower&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-8"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-printing-character-1"></a>
<dl>
<dt><a name="index-iswprint"></a><u>Function:</u> int <b>iswprint</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a printing character.  Printing characters
include all the graphic characters, plus the space (&lsquo;<samp> </samp>&rsquo;) character.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;print&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-9"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-punctuation-character-1"></a>
<dl>
<dt><a name="index-iswpunct"></a><u>Function:</u> int <b>iswpunct</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a punctuation character.
This means any printing character that is not alphanumeric or a space
character.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;punct&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-10"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-whitespace-character-1"></a>
<dl>
<dt><a name="index-iswspace"></a><u>Function:</u> int <b>iswspace</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a <em>whitespace</em> character.  In the standard
<code>&quot;C&quot;</code> locale, <code>iswspace</code> returns true for only the standard
whitespace characters:
</p>
<dl compact="compact">
<dt> <code>L' '</code></dt>
<dd><p>space
</p>
</dd>
<dt> <code>L'\f'</code></dt>
<dd><p>formfeed
</p>
</dd>
<dt> <code>L'\n'</code></dt>
<dd><p>newline
</p>
</dd>
<dt> <code>L'\r'</code></dt>
<dd><p>carriage return
</p>
</dd>
<dt> <code>L'\t'</code></dt>
<dd><p>horizontal tab
</p>
</dd>
<dt> <code>L'\v'</code></dt>
<dd><p>vertical tab
</p></dd>
</dl>

<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;space&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-11"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-upper_002dcase-character-1"></a>
<dl>
<dt><a name="index-iswupper"></a><u>Function:</u> int <b>iswupper</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is an upper-case letter.  The letter need not be
from the Latin alphabet, any alphabet representable is valid.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;upper&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-12"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<a name="index-hexadecimal-digit-character-1"></a>
<dl>
<dt><a name="index-iswxdigit"></a><u>Function:</u> int <b>iswxdigit</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a hexadecimal digit.
Hexadecimal digits include the normal decimal digits &lsquo;<samp>0</samp>&rsquo; through
&lsquo;<samp>9</samp>&rsquo; and the letters &lsquo;<samp>A</samp>&rsquo; through &lsquo;<samp>F</samp>&rsquo; and
&lsquo;<samp>a</samp>&rsquo; through &lsquo;<samp>f</samp>&rsquo;.
</p>
<p>This function can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">iswctype (wc, wctype (&quot;xdigit&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-13"></a>
<p>It is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<p>The GNU C library also provides a function which is not defined in the
ISO C standard but which is available as a version for single byte
characters as well.
</p>
<a name="index-blank-character-1"></a>
<dl>
<dt><a name="index-iswblank"></a><u>Function:</u> int <b>iswblank</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>Returns true if <var>wc</var> is a blank character; that is, a space or a tab.
This function was originally a GNU extension, but was added in ISO C99.
It is declared in &lsquo;<tt>wchar.h</tt>&rsquo;.
</p></dd></dl>

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<a name="Notes-on-using-the-wide-character-classes"></a>
<h2 class="section">4.4 Notes on using the wide character classes</h2>

<p>The first note is probably not astonishing but still occasionally a
cause of problems.  The <code>isw<var>XXX</var></code> functions can be implemented
using macros and in fact, the GNU C library does this.  They are still
available as real functions but when the &lsquo;<tt>wctype.h</tt>&rsquo; header is
included the macros will be used.  This is the same as the
<code>char</code> type versions of these functions.
</p>
<p>The second note covers something new.  It can be best illustrated by a
(real-world) example.  The first piece of code is an excerpt from the
original code.  It is truncated a bit but the intention should be clear.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">int
is_in_class (int c, const char *class)
{
  if (strcmp (class, &quot;alnum&quot;) == 0)
    return isalnum (c);
  if (strcmp (class, &quot;alpha&quot;) == 0)
    return isalpha (c);
  if (strcmp (class, &quot;cntrl&quot;) == 0)
    return iscntrl (c);
  &hellip;
  return 0;
}
</pre></td></tr></table>

<p>Now, with the <code>wctype</code> and <code>iswctype</code> you can avoid the
<code>if</code> cascades, but rewriting the code as follows is wrong:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">int
is_in_class (int c, const char *class)
{
  wctype_t desc = wctype (class);
  return desc ? iswctype ((wint_t) c, desc) : 0;
}
</pre></td></tr></table>

<p>The problem is that it is not guaranteed that the wide character
representation of a single-byte character can be found using casting.
In fact, usually this fails miserably.  The correct solution to this
problem is to write the code as follows:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">int
is_in_class (int c, const char *class)
{
  wctype_t desc = wctype (class);
  return desc ? iswctype (btowc (c), desc) : 0;
}
</pre></td></tr></table>

<p>See section <a href="libc_6.html#Converting-a-Character">Converting Single Characters</a>, for more information on <code>btowc</code>.
Note that this change probably does not improve the performance
of the program a lot since the <code>wctype</code> function still has to make
the string comparisons.  It gets really interesting if the
<code>is_in_class</code> function is called more than once for the
same class name.  In this case the variable <var>desc</var> could be computed
once and reused for all the calls.  Therefore the above form of the
function is probably not the final one.
</p>

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<a name="Mapping-of-wide-characters_002e"></a>
<h2 class="section">4.5 Mapping of wide characters.</h2>

<p>The classification functions are also generalized by the ISO C
standard.  Instead of just allowing the two standard mappings, a
locale can contain others.  Again, the <code>localedef</code> program
already supports generating such locale data files.
</p>
<dl>
<dt><a name="index-wctrans_005ft"></a><u>Data Type:</u> <b>wctrans_t</b></dt>
<dd><p>This data type is defined as a scalar type which can hold a value
representing the locale-dependent character mapping.  There is no way to
construct such a value apart from using the return value of the
<code>wctrans</code> function.
</p>
<a name="index-wctype_002eh-14"></a>
<p>This type is defined in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-wctrans"></a><u>Function:</u> wctrans_t <b>wctrans</b><i> (const char *<var>property</var>)</i></dt>
<dd><p>The <code>wctrans</code> function has to be used to find out whether a named
mapping is defined in the current locale selected for the
<code>LC_CTYPE</code> category.  If the returned value is non-zero, you can use
it afterwards in calls to <code>towctrans</code>.  If the return value is
zero no such mapping is known in the current locale.
</p>
<p>Beside locale-specific mappings there are two mappings which are
guaranteed to be available in every locale:
</p>
<table>
<tr><td width="50%"><code>&quot;tolower&quot;</code></td><td width="50%"><code>&quot;toupper&quot;</code></td></tr>
</table>

<a name="index-wctype_002eh-15"></a>
<p>These functions are declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-towctrans"></a><u>Function:</u> wint_t <b>towctrans</b><i> (wint_t <var>wc</var>, wctrans_t <var>desc</var>)</i></dt>
<dd><p><code>towctrans</code> maps the input character <var>wc</var>
according to the rules of the mapping for which <var>desc</var> is a
descriptor, and returns the value it finds.  <var>desc</var> must be
obtained by a successful call to <code>wctrans</code>.
</p>
<a name="index-wctype_002eh-16"></a>
<p>This function is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<p>For the generally available mappings, the ISO C standard defines
convenient shortcuts so that it is not necessary to call <code>wctrans</code>
for them.
</p>
<dl>
<dt><a name="index-towlower"></a><u>Function:</u> wint_t <b>towlower</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>If <var>wc</var> is an upper-case letter, <code>towlower</code> returns the corresponding
lower-case letter.  If <var>wc</var> is not an upper-case letter,
<var>wc</var> is returned unchanged.
</p>
<p><code>towlower</code> can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">towctrans (wc, wctrans (&quot;tolower&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-17"></a>
<p>This function is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-towupper"></a><u>Function:</u> wint_t <b>towupper</b><i> (wint_t <var>wc</var>)</i></dt>
<dd><p>If <var>wc</var> is a lower-case letter, <code>towupper</code> returns the corresponding
upper-case letter.  Otherwise <var>wc</var> is returned unchanged.
</p>
<p><code>towupper</code> can be implemented using
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">towctrans (wc, wctrans (&quot;toupper&quot;))
</pre></td></tr></table>

<a name="index-wctype_002eh-18"></a>
<p>This function is declared in &lsquo;<tt>wctype.h</tt>&rsquo;.
</p></dd></dl>

<p>The same warnings given in the last section for the use of the wide
character classification functions apply here.  It is not possible to
simply cast a <code>char</code> type value to a <code>wint_t</code> and use it as an
argument to <code>towctrans</code> calls.
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