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### 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.