#error

#error "This code should not compile"
The #error macro allows you to make compilation fail and issue a statement that will appear in the list of compilation errors. It is most useful when combined with #if/#elif/#else to fail compilation if some condition is not true. For example:
#ifndef __unix__ // __unix__ is typically defined when targetting Unix 
#error "Only Unix is supported"
#endif


Related

C preprocessor tutorial