Gcc/10.2.0/cpp/Traditional-miscellany
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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.
