Is there a portable way to print a message from the C preprocessor?

The warning directive is probably the closest you'll get, but it's not entirely platform-independent:

#warning "C Preprocessor got here!"

AFAIK this works on most compilers except MSVC, on which you'll have to use a pragma directive:

#pragma message ( "C Preprocessor got here!" )

The following are supported by MSVC, and GCC.

#pragma message("stuff")
#pragma message "stuff"

Clang has begun adding support recently, see here for more.

You might want to try: #pragma message("Hello World!")