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<title>The C Preprocessor: Stringizing</title>

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<a name="Stringizing"></a>
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Next: <a href="Concatenation.html#Concatenation" accesskey="n" rel="next">Concatenation</a>, Previous: <a href="Macro-Arguments.html#Macro-Arguments" accesskey="p" rel="prev">Macro Arguments</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Stringizing-1"></a>
<h3 class="section">3.4 Stringizing</h3>
<a name="index-stringizing"></a>
<a name="index-_0023-operator"></a>

<p>Sometimes you may want to convert a macro argument into a string
constant.  Parameters are not replaced inside string constants, but you
can use the &lsquo;<samp>#</samp>&rsquo; preprocessing operator instead.  When a macro
parameter is used with a leading &lsquo;<samp>#</samp>&rsquo;, the preprocessor replaces it
with the literal text of the actual argument, converted to a string
constant.  Unlike normal parameter replacement, the argument is not
macro-expanded first.  This is called <em>stringizing</em>.
</p>
<p>There is no way to combine an argument with surrounding text and
stringize it all together.  Instead, you can write a series of adjacent
string constants and stringized arguments.  The preprocessor
replaces the stringized arguments with string constants.  The C
compiler then combines all the adjacent string constants into one
long string.
</p>
<p>Here is an example of a macro definition that uses stringizing:
</p>
<div class="smallexample">
<pre class="smallexample">#define WARN_IF(EXP) \
do { if (EXP) \
        fprintf (stderr, &quot;Warning: &quot; #EXP &quot;\n&quot;); } \
while (0)
WARN_IF (x == 0);
     &rarr; do { if (x == 0)
           fprintf (stderr, &quot;Warning: &quot; &quot;x == 0&quot; &quot;\n&quot;); } while (0);
</pre></div>

<p>The argument for <code>EXP</code> is substituted once, as-is, into the
<code>if</code> statement, and once, stringized, into the argument to
<code>fprintf</code>.  If <code>x</code> were a macro, it would be expanded in the
<code>if</code> statement, but not in the string.
</p>
<p>The <code>do</code> and <code>while (0)</code> are a kludge to make it possible to
write <code>WARN_IF (<var>arg</var>);</code>, which the resemblance of
<code>WARN_IF</code> to a function would make C programmers want to do; see
<a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">Swallowing the Semicolon</a>.
</p>
<p>Stringizing in C involves more than putting double-quote characters
around the fragment.  The preprocessor backslash-escapes the quotes
surrounding embedded string constants, and all backslashes within string and
character constants, in order to get a valid C string constant with the
proper contents.  Thus, stringizing <code>p&nbsp;=&nbsp;&quot;foo\n&quot;;<!-- /@w --></code> results in
<tt>&quot;p&nbsp;=&nbsp;\&quot;foo\\n\&quot;;&quot;<!-- /@w --></tt>.  However, backslashes that are not inside string
or character constants are not duplicated: &lsquo;<samp>\n</samp>&rsquo; by itself
stringizes to <tt>&quot;\n&quot;</tt>.
</p>
<p>All leading and trailing whitespace in text being stringized is
ignored.  Any sequence of whitespace in the middle of the text is
converted to a single space in the stringized result.  Comments are
replaced by whitespace long before stringizing happens, so they
never appear in stringized text.
</p>
<p>There is no way to convert a macro argument into a character constant.
</p>
<p>If you want to stringize the result of expansion of a macro argument,
you have to use two levels of macros.
</p>
<div class="smallexample">
<pre class="smallexample">#define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)
     &rarr; &quot;foo&quot;
xstr (foo)
     &rarr; xstr (4)
     &rarr; str (4)
     &rarr; &quot;4&quot;
</pre></div>

<p><code>s</code> is stringized when it is used in <code>str</code>, so it is not
macro-expanded first.  But <code>s</code> is an ordinary argument to
<code>xstr</code>, so it is completely macro-expanded before <code>xstr</code>
itself is expanded (see <a href="Argument-Prescan.html#Argument-Prescan">Argument Prescan</a>).  Therefore, by the time
<code>str</code> gets to its argument, it has already been macro-expanded.
</p>
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