cpp.info: Diagnostics

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   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."
   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)
   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.