Here are some things to be aware of when using the traditional
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.