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## 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.