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(FOO) && defined(BAR)
#error "BAR requires FOO."
#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.