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<h3 class="section">3.1 Object-like Macros</h3>
<p><a name="index-object_002dlike-macro-41"></a><a name="index-symbolic-constants-42"></a><a name="index-manifest-constants-43"></a>
An <dfn>object-like macro</dfn> is a simple identifier which will be replaced
by a code fragment. It is called object-like because it looks like a
data object in code that uses it. They are most commonly used to give
symbolic names to numeric constants.
<p><a name="index-g_t_0023define-44"></a>You create macros with the ‘<samp><span class="samp">#define</span></samp>’ directive. ‘<samp><span class="samp">#define</span></samp>’ is
followed by the name of the macro and then the token sequence it should
be an abbreviation for, which is variously referred to as the macro's
<dfn>body</dfn>, <dfn>expansion</dfn> or <dfn>replacement list</dfn>. For example,
<pre class="smallexample"> #define BUFFER_SIZE 1024
</pre>
<p class="noindent">defines a macro named <code>BUFFER_SIZE</code> as an abbreviation for the
token <code>1024</code>. If somewhere after this ‘<samp><span class="samp">#define</span></samp>’ directive
there comes a C statement of the form
<pre class="smallexample"> foo = (char *) malloc (BUFFER_SIZE);
</pre>
<p class="noindent">then the C preprocessor will recognize and <dfn>expand</dfn> the macro
<code>BUFFER_SIZE</code>. The C compiler will see the same tokens as it would
if you had written
<pre class="smallexample"> foo = (char *) malloc (1024);
</pre>
<p>By convention, macro names are written in uppercase. Programs are
easier to read when it is possible to tell at a glance which names are
macros.
<p>The macro's body ends at the end of the ‘<samp><span class="samp">#define</span></samp>’ line. You may
continue the definition onto multiple lines, if necessary, using
backslash-newline. When the macro is expanded, however, it will all
come out on one line. For example,
<pre class="smallexample"> #define NUMBERS 1, \
2, \
3
int x[] = { NUMBERS };
==> int x[] = { 1, 2, 3 };
</pre>
<p class="noindent">The most common visible consequence of this is surprising line numbers
in error messages.
<p>There is no restriction on what can go in a macro body provided it
decomposes into valid preprocessing tokens. Parentheses need not
balance, and the body need not resemble valid C code. (If it does not,
you may get error messages from the C compiler when you use the macro.)
<p>The C preprocessor scans your program sequentially. Macro definitions
take effect at the place you write them. Therefore, the following input
to the C preprocessor
<pre class="smallexample"> foo = X;
#define X 4
bar = X;
</pre>
<p class="noindent">produces
<pre class="smallexample"> foo = X;
bar = 4;
</pre>
<p>When the preprocessor expands a macro name, the macro's expansion
replaces the macro invocation, then the expansion is examined for more
macros to expand. For example,
<pre class="smallexample"> #define TABLESIZE BUFSIZE
#define BUFSIZE 1024
TABLESIZE
==> BUFSIZE
==> 1024
</pre>
<p class="noindent"><code>TABLESIZE</code> is expanded first to produce <code>BUFSIZE</code>, then that
macro is expanded to produce the final result, <code>1024</code>.
<p>Notice that <code>BUFSIZE</code> was not defined when <code>TABLESIZE</code> was
defined. The ‘<samp><span class="samp">#define</span></samp>’ for <code>TABLESIZE</code> uses exactly the
expansion you specify—in this case, <code>BUFSIZE</code>—and does not
check to see whether it too contains macro names. Only when you
<em>use</em> <code>TABLESIZE</code> is the result of its expansion scanned for
more macro names.
<p>This makes a difference if you change the definition of <code>BUFSIZE</code>
at some point in the source file. <code>TABLESIZE</code>, defined as shown,
will always expand using the definition of <code>BUFSIZE</code> that is
currently in effect:
<pre class="smallexample"> #define BUFSIZE 1020
#define TABLESIZE BUFSIZE
#undef BUFSIZE
#define BUFSIZE 37
</pre>
<p class="noindent">Now <code>TABLESIZE</code> expands (in two stages) to <code>37</code>.
<p>If the expansion of a macro contains its own name, either directly or
via intermediate macros, it is not expanded again when the expansion is
examined for more macros. This prevents infinite recursion.
See <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>, for the precise details.
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