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<title>The C Preprocessor: Object-like Macros</title>
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<a name="Object_002dlike-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-like Macros</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="Object_002dlike-Macros-1"></a>
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<h3 class="section">3.1 Object-like Macros</h3>
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<a name="index-object_002dlike-macro"></a>
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<a name="index-symbolic-constants"></a>
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<a name="index-manifest-constants"></a>
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<p>An <em>object-like macro</em> is a simple identifier which will be replaced
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by a code fragment. It is called object-like because it looks like a
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data object in code that uses it. They are most commonly used to give
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symbolic names to numeric constants.
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</p>
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<a name="index-_0023define"></a>
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<p>You create macros with the ‘<samp>#define</samp>’ directive. ‘<samp>#define</samp>’ is
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followed by the name of the macro and then the token sequence it should
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be an abbreviation for, which is variously referred to as the macro’s
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<em>body</em>, <em>expansion</em> or <em>replacement list</em>. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define BUFFER_SIZE 1024
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</pre></div>
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<p>defines a macro named <code>BUFFER_SIZE</code> as an abbreviation for the
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token <code>1024</code>. If somewhere after this ‘<samp>#define</samp>’ directive
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there comes a C statement of the form
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</p>
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<div class="smallexample">
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<pre class="smallexample">foo = (char *) malloc (BUFFER_SIZE);
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</pre></div>
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<p>then the C preprocessor will recognize and <em>expand</em> the macro
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<code>BUFFER_SIZE</code>. The C compiler will see the same tokens as it would
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if you had written
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</p>
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<div class="smallexample">
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<pre class="smallexample">foo = (char *) malloc (1024);
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</pre></div>
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<p>By convention, macro names are written in uppercase. Programs are
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easier to read when it is possible to tell at a glance which names are
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macros.
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</p>
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<p>The macro’s body ends at the end of the ‘<samp>#define</samp>’ line. You may
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continue the definition onto multiple lines, if necessary, using
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backslash-newline. When the macro is expanded, however, it will all
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come out on one line. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define NUMBERS 1, \
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2, \
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3
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int x[] = { NUMBERS };
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→ int x[] = { 1, 2, 3 };
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</pre></div>
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<p>The most common visible consequence of this is surprising line numbers
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in error messages.
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</p>
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<p>There is no restriction on what can go in a macro body provided it
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decomposes into valid preprocessing tokens. Parentheses need not
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balance, and the body need not resemble valid C code. (If it does not,
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you may get error messages from the C compiler when you use the macro.)
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</p>
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<p>The C preprocessor scans your program sequentially. Macro definitions
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take effect at the place you write them. Therefore, the following input
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to the C preprocessor
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</p>
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<div class="smallexample">
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<pre class="smallexample">foo = X;
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#define X 4
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bar = X;
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</pre></div>
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<p>produces
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</p>
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<div class="smallexample">
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<pre class="smallexample">foo = X;
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bar = 4;
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</pre></div>
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<p>When the preprocessor expands a macro name, the macro’s expansion
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replaces the macro invocation, then the expansion is examined for more
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macros to expand. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define TABLESIZE BUFSIZE
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#define BUFSIZE 1024
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TABLESIZE
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→ BUFSIZE
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→ 1024
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</pre></div>
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<p><code>TABLESIZE</code> is expanded first to produce <code>BUFSIZE</code>, then that
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macro is expanded to produce the final result, <code>1024</code>.
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</p>
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<p>Notice that <code>BUFSIZE</code> was not defined when <code>TABLESIZE</code> was
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defined. The ‘<samp>#define</samp>’ for <code>TABLESIZE</code> uses exactly the
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expansion you specify—in this case, <code>BUFSIZE</code>—and does not
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check to see whether it too contains macro names. Only when you
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<em>use</em> <code>TABLESIZE</code> is the result of its expansion scanned for
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more macro names.
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</p>
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<p>This makes a difference if you change the definition of <code>BUFSIZE</code>
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at some point in the source file. <code>TABLESIZE</code>, defined as shown,
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will always expand using the definition of <code>BUFSIZE</code> that is
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currently in effect:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define BUFSIZE 1020
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#define TABLESIZE BUFSIZE
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#undef BUFSIZE
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#define BUFSIZE 37
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</pre></div>
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<p>Now <code>TABLESIZE</code> expands (in two stages) to <code>37</code>.
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</p>
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<p>If the expansion of a macro contains its own name, either directly or
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via intermediate macros, it is not expanded again when the expansion is
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examined for more macros. This prevents infinite recursion.
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See <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>, for the precise details.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-like Macros</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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