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<h3 class="section">3.4 Stringification</h3>
<p><a name="index-stringification-50"></a><a name="index-g_t_0040samp_007b_0023_007d-operator-51"></a>
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 ‘<samp><span class="samp">#</span></samp>’ preprocessing operator instead. When a macro
parameter is used with a leading ‘<samp><span class="samp">#</span></samp>’, 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 <dfn>stringification</dfn>.
<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>Here is an example of a macro definition that uses stringification:
<pre class="smallexample"> #define WARN_IF(EXP) \
do { if (EXP) \
fprintf (stderr, "Warning: " #EXP "\n"); } \
while (0)
WARN_IF (x == 0);
==> do { if (x == 0)
fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0);
</pre>
<p class="noindent">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>The <code>do</code> and <code>while (0)</code> are a kludge to make it possible to
write <code>WARN_IF (</code><var>arg</var><code>);</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>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 = "foo\n";<!-- /@w --></code> results in
<tt>"p = \"foo\\n\";"<!-- /@w --></tt>. However, backslashes that are not inside string
or character constants are not duplicated: ‘<samp><span class="samp">\n</span></samp>’ by itself
stringifies to <tt>"\n"</tt>.
<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>There is no way to convert a macro argument into a character constant.
<p>If you want to stringify the result of expansion of a macro argument,
you have to use two levels of macros.
<pre class="smallexample"> #define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)
==> "foo"
xstr (foo)
==> xstr (4)
==> str (4)
==> "4"
</pre>
<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.
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