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1.5.6 The `#error' and `#warning' Directives

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. The line must consist of complete tokens.

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 1.4.3.2 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.