Preprocessing directives

• How sequences in both forms of header names are mapped to headers or external source file names (6.4.7).
• Whether the value of a character constant in a constant expression that controls conditional inclusion matches the value of the same character constant in the execution character set (6.10.1).
• Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (6.10.1).
• The places that are searched for an included <> delimited header, and how the places are specified or the header is identified (6.10.2).
• How the named source file is searched for in an included "" delimited header (6.10.2).
• The method by which preprocessing tokens (possibly resulting from macro expansion) in a #include directive are combined into a header name (6.10.2).
• The nesting limit for #include processing (6.10.2).

  GCC imposes a limit of 200 nested #includes.

• Whether the # operator inserts a \ character before the \ character that begins a universal character name in a character constant or string literal (6.10.3.2).
• The behavior on each recognized non-STDC #pragma directive (6.10.6).
• The definitions for __DATE__ and __TIME__ when respectively, the date and time of translation are not available (6.10.8).

  If the date and time are not available, __DATE__ expands to "??? ?? ????" and __TIME__ expands to "??:??:??".