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



   <p>The directive <code>#error</code> causes the preprocessor to report a fatal

error.  The tokens forming the rest of the line following <code>#error</code>

are used as the error message.



   <p>You would use <code>#error</code> 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="example">     #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 <code>#error</code>.  For example,



<pre class="example">     #if !defined(UNALIGNED_INT_ASM_OP) &amp;&amp; defined(DWARF2_DEBUGGING_INFO)

     #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."

     #endif

     </pre>



   <p>The directive <code>#warning</code> is like <code>#error</code>, but causes the

preprocessor to issue a warning and continue preprocessing.  The tokens

following <code>#warning</code> are used as the warning message.



   <p>You might use <code>#warning</code> in obsolete header files, with a message

directing the user to the header file which should be used instead.



   <p>Neither <code>#error</code> nor <code>#warning</code> 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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