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(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.