Node:Diagnostics, Next:Line Control, Previous:Conditionals, Up:Top

## 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(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.