10.3 Traditional miscellany
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.