## 5 Diagnostics

The directive ``#error` ' causes the preprocessor to report a fatal error. The tokens forming the rest of the line following ``#error` ' are used as the error message.

You would use ``#error` ' 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

         #ifdef __vax__
         #error "Won't work on VAXen.  See comments at get_last_object."
         #endif

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 ``#error` '. For example,

         #if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
         #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
         #endif

The directive ``#warning` ' is like ``#error` ', but causes the preprocessor to issue a warning and continue preprocessing. The tokens following ``#warning` ' are used as the warning message.

You might use ``#warning` ' in obsolete header files, with a message directing the user to the header file which should be used instead.

Neither ``#error` ' nor ``#warning` ' 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.