C251 User's Guide

#warning

The #warning directive causes the preprocessor to emit a warning. The rest of the line is the warning message.

#warning "message"

You may use #warning inside a condition that detects an error condition. For example, the following outputs a warning message if __KEIL__ is not defined.

#ifndef __KEIL__
#warning "This code probably will not work without the Keil Compiler."
#endif