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<title>GNU Compiler Collection (GCC) Internals: Target Structure</title>

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<a name="Target-Structure"></a>
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<a name="The-Global-targetm-Variable"></a>
<h3 class="section">17.1 The Global <code>targetm</code> Variable</h3>
<a name="index-target-hooks"></a>
<a name="index-target-functions"></a>

<dl>
<dt><a name="index-targetm"></a>Variable: <em>struct gcc_target</em> <strong>targetm</strong></dt>
<dd><p>The target <samp>.c</samp> file must define the global <code>targetm</code> variable
which contains pointers to functions and data relating to the target
machine.  The variable is declared in <samp>target.h</samp>;
<samp>target-def.h</samp> defines the macro <code>TARGET_INITIALIZER</code> which is
used to initialize the variable, and macros for the default initializers
for elements of the structure.  The <samp>.c</samp> file should override those
macros for which the default definition is inappropriate.  For example:
</p><div class="smallexample">
<pre class="smallexample">#include &quot;target.h&quot;
#include &quot;target-def.h&quot;

/* <span class="roman">Initialize the GCC target structure.</span>  */

#undef TARGET_COMP_TYPE_ATTRIBUTES
#define TARGET_COMP_TYPE_ATTRIBUTES <var>machine</var>_comp_type_attributes

struct gcc_target targetm = TARGET_INITIALIZER;
</pre></div>
</dd></dl>

<p>Where a macro should be defined in the <samp>.c</samp> file in this manner to
form part of the <code>targetm</code> structure, it is documented below as a
&ldquo;Target Hook&rdquo; with a prototype.  Many macros will change in future
from being defined in the <samp>.h</samp> file to being part of the
<code>targetm</code> structure.
</p>
<p>Similarly, there is a <code>targetcm</code> variable for hooks that are
specific to front ends for C-family languages, documented as &ldquo;C
Target Hook&rdquo;.  This is declared in <samp>c-family/c-target.h</samp>, the
initializer <code>TARGETCM_INITIALIZER</code> in
<samp>c-family/c-target-def.h</samp>.  If targets initialize <code>targetcm</code>
themselves, they should set <code>target_has_targetcm=yes</code> in
<samp>config.gcc</samp>; otherwise a default definition is used.
</p>
<p>Similarly, there is a <code>targetm_common</code> variable for hooks that
are shared between the compiler driver and the compilers proper,
documented as &ldquo;Common Target Hook&rdquo;.  This is declared in
<samp>common/common-target.h</samp>, the initializer
<code>TARGETM_COMMON_INITIALIZER</code> in
<samp>common/common-target-def.h</samp>.  If targets initialize
<code>targetm_common</code> themselves, they should set
<code>target_has_targetm_common=yes</code> in <samp>config.gcc</samp>; otherwise a
default definition is used.
</p>


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