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<h2 class="chapter">5 Diagnostics</h2>

<p><a name="index-diagnostic-93"></a><a name="index-reporting-errors-94"></a><a name="index-reporting-warnings-95"></a>
<a name="index-g_t_0023error-96"></a>The directive &lsquo;<samp><span class="samp">#error</span></samp>&rsquo; causes the preprocessor to report a fatal
error.  The tokens forming the rest of the line following &lsquo;<samp><span class="samp">#error</span></samp>&rsquo;
are used as the error message.

   <p>You would use &lsquo;<samp><span class="samp">#error</span></samp>&rsquo; inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a VAX, you might write

<pre class="smallexample">     #ifdef __vax__
     #error "Won't work on VAXen.  See comments at get_last_object."
     #endif
</pre>
   <p>If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with &lsquo;<samp><span class="samp">#error</span></samp>&rsquo;.  For example,

<pre class="smallexample">     #if !defined(UNALIGNED_INT_ASM_OP) &amp;&amp; defined(DWARF2_DEBUGGING_INFO)
     #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
     #endif
</pre>
   <p><a name="index-g_t_0023warning-97"></a>The directive &lsquo;<samp><span class="samp">#warning</span></samp>&rsquo; is like &lsquo;<samp><span class="samp">#error</span></samp>&rsquo;, but causes the
preprocessor to issue a warning and continue preprocessing.  The tokens
following &lsquo;<samp><span class="samp">#warning</span></samp>&rsquo; are used as the warning message.

   <p>You might use &lsquo;<samp><span class="samp">#warning</span></samp>&rsquo; in obsolete header files, with a message
directing the user to the header file which should be used instead.

   <p>Neither &lsquo;<samp><span class="samp">#error</span></samp>&rsquo; nor &lsquo;<samp><span class="samp">#warning</span></samp>&rsquo; macro-expands its argument. 
Internal whitespace sequences are each replaced with a single space. 
The line must consist of complete tokens.  It is wisest to make the
argument of these directives be a single string constant; this avoids
problems with apostrophes and the like.

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