The directive `#error' causes the preprocessor to report a fatal error. The rest of the line that follows `#error' is 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
See section Nonstandard Predefined Macros, for why this works.
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 HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
|| HASH_TABLE_SIZE % 5 == 0
#error HASH_TABLE_SIZE should not be divisible by a small prime
#endif
The directive `#warning' is like the directive `#error', but causes the preprocessor to issue a warning and continue preprocessing. The rest of the line that follows `#warning' is 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.