This preprocessor directive can be used to make a file not compile with your own error message saying what's wrong

Code:

#error Custom error message saying what is wrong

Usage:

int foo(int bar) {

     #error Function foo not yet implemented

}

Extensions:

This can be used with #if to check properties of the environment

#if sizeof(int) < 4

     #error sizeof Int is too small, must be at least 4 bytes

#endif

Sources:

https://www.securecoding.cert.org/confluence/display/seccode/DCL03-A.+Use+a+static+assertion+to+test+the+value+of+a+constant+expression