toolchain/gcc-linaro-6.3.1-2017.02-x8.../share/doc/cpp/Stringification.html

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<a name="Stringification"></a>
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<a name="Stringification-1"></a>
<h3 class="section">3.4 Stringification</h3>
<a name="index-stringification"></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>stringification</em>.
</p>
<p>There is no way to combine an argument with surrounding text and
stringify it all together.  Instead, you can write a series of adjacent
string constants and stringified arguments.  The preprocessor will
replace the stringified arguments with string constants.  The C
compiler will then combine all the adjacent string constants into one
long string.
</p>
<p>Here is an example of a macro definition that uses stringification:
</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, stringified, 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>Stringification 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, stringifying <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
stringifies to <tt>&quot;\n&quot;</tt>.
</p>
<p>All leading and trailing whitespace in text being stringified is
ignored.  Any sequence of whitespace in the middle of the text is
converted to a single space in the stringified result.  Comments are
replaced by whitespace long before stringification happens, so they
never appear in stringified text.
</p>
<p>There is no way to convert a macro argument into a character constant.
</p>
<p>If you want to stringify 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 stringified 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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