| User-defined 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."
    #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.