### 10.3 Traditional miscellany

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.