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<title>The C Preprocessor: Stringification</title>
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<a name="Stringification"></a>
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<p>
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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> [<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>
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<a name="Stringification-1"></a>
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<h3 class="section">3.4 Stringification</h3>
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<a name="index-stringification"></a>
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<a name="index-_0023-operator"></a>
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<p>Sometimes you may want to convert a macro argument into a string
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constant. Parameters are not replaced inside string constants, but you
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can use the ‘<samp>#</samp>’ preprocessing operator instead. When a macro
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parameter is used with a leading ‘<samp>#</samp>’, the preprocessor replaces it
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with the literal text of the actual argument, converted to a string
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constant. Unlike normal parameter replacement, the argument is not
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macro-expanded first. This is called <em>stringification</em>.
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</p>
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<p>There is no way to combine an argument with surrounding text and
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stringify it all together. Instead, you can write a series of adjacent
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string constants and stringified arguments. The preprocessor will
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replace the stringified arguments with string constants. The C
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compiler will then combine all the adjacent string constants into one
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long string.
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</p>
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<p>Here is an example of a macro definition that uses stringification:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define WARN_IF(EXP) \
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do { if (EXP) \
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fprintf (stderr, "Warning: " #EXP "\n"); } \
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while (0)
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WARN_IF (x == 0);
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→ do { if (x == 0)
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fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0);
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</pre></div>
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<p>The argument for <code>EXP</code> is substituted once, as-is, into the
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<code>if</code> statement, and once, stringified, into the argument to
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<code>fprintf</code>. If <code>x</code> were a macro, it would be expanded in the
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<code>if</code> statement, but not in the string.
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</p>
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<p>The <code>do</code> and <code>while (0)</code> are a kludge to make it possible to
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write <code>WARN_IF (<var>arg</var>);</code>, which the resemblance of
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<code>WARN_IF</code> to a function would make C programmers want to do; see
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<a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">Swallowing the Semicolon</a>.
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</p>
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<p>Stringification in C involves more than putting double-quote characters
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around the fragment. The preprocessor backslash-escapes the quotes
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surrounding embedded string constants, and all backslashes within string and
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character constants, in order to get a valid C string constant with the
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proper contents. Thus, stringifying <code>p = "foo\n";<!-- /@w --></code> results in
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<tt>"p = \"foo\\n\";"<!-- /@w --></tt>. However, backslashes that are not inside string
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or character constants are not duplicated: ‘<samp>\n</samp>’ by itself
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stringifies to <tt>"\n"</tt>.
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</p>
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<p>All leading and trailing whitespace in text being stringified is
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ignored. Any sequence of whitespace in the middle of the text is
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converted to a single space in the stringified result. Comments are
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replaced by whitespace long before stringification happens, so they
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never appear in stringified text.
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</p>
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<p>There is no way to convert a macro argument into a character constant.
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</p>
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<p>If you want to stringify the result of expansion of a macro argument,
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you have to use two levels of macros.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define xstr(s) str(s)
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#define str(s) #s
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#define foo 4
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str (foo)
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→ "foo"
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xstr (foo)
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→ xstr (4)
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→ str (4)
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→ "4"
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</pre></div>
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<p><code>s</code> is stringified when it is used in <code>str</code>, so it is not
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macro-expanded first. But <code>s</code> is an ordinary argument to
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<code>xstr</code>, so it is completely macro-expanded before <code>xstr</code>
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itself is expanded (see <a href="Argument-Prescan.html#Argument-Prescan">Argument Prescan</a>). Therefore, by the time
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<code>str</code> gets to its argument, it has already been macro-expanded.
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</p>
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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> [<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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