The #error directive 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 use the following input.
#ifdef __vax__ #error Won’t work on Vaxen. See comments at get_last_object. #endifSee Nonstandard predefined macros for a description of 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 clarification, see the following 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 #endifThe #warning directive is like the #error directive, 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 4c file which should instead be used. 0