6.12 Pre-Processing Directives

ISO Standard Implementation
“How sequences in both forms of header names are mapped to headers or external source file names (C90 6.1.7, C99 6.4.7).”
The character sequence between the delimiters is considered to be a string which is a file name for the host environment.
“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 (C90 6.8.1, C99 6.10.1).”
Yes.
“Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (C90 6.8.1, C99 6.10.1).”
Yes.
“The places that are searched for an included < > delimited header and how the places are specified or the header is identified (C90 6.8.2, C99 6.10.2).”
The preprocessor searches any directory specified using the -I option, then, provided the -nostdinc option has not been used, the standard compiler include directory, <install directory>/avr/avr/ include.
“How the named source file is searched for in an included " " delimited header (C90 6.8.2, C99 6.10.2).”
The compiler first searches for the named file in the directory containing the including file, then the directories which are searched for a < > delimited header.
“The method by which preprocessing tokens are combined into a header name (C90 6.8.2, C99 6.10.2).”
All tokens, including whitespace, are considered part of the header file name. Macro expansion is not performed on tokens inside the delimiters.
“The nesting limit for #include processing (C90 6.8.2, C99 6.10.2).”
No limit.
“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).”
No.
“The behavior on each recognized non-STDC #pragma directive (C90 6.8.6, C99 6.10.6).”
See the section on Pragma Directives.
“The definitions for __DATE__ and __TIME__ when respectively, the date and time of translation are not available (C90 6.8.8, C99 6.10.8).”
The date and time of translation are always available.