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.