See Implementation-defined behavior (The C Preprocessor), for details of these aspects of
implementation-defined behavior.
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).
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).
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).
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).
How the named source file is searched for in an included ""
delimited header (C90 6.8.2, C99 6.10.2).
The method by which preprocessing tokens (possibly resulting from
macro expansion) in a #include directive are combined into a header
name (C90 6.8.2, C99 6.10.2).
The nesting limit for #include processing (C90 6.8.2, C99
6.10.2).
Whether the # operator inserts a \ character before
the \ character that begins a universal character name in a
character constant or string literal (C99 6.10.3.2).
The behavior on each recognized non-STDC #pragma
directive (C90 6.8.6, C99 6.10.6).
See Pragmas (The C Preprocessor), for details of
pragmas accepted by GCC on all targets. See Pragmas Accepted by GCC, for details of target-specific pragmas.
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).
