Here are some things to be aware of when using the traditional preprocessor.
Preprocessing directives are recognized only when their leading # appears in the first column. There can be no whitespace between the beginning of the line and the #, but whitespace can follow the #.
A true traditional C preprocessor does not recognize #error or #pragma, and may not recognize #elif. CPP supports all the directives in traditional mode that it supports in ISO mode, including extensions, with the exception that the effects of #pragma GCC poison are undefined.
__STDC__ is not defined.
If you use digraphs the behavior is undefined.
If a line that looks like a directive appears within macro arguments, the behavior is undefined.