4.13 Preprocessing directives

See Implementation-defined behavior, for details of these aspects of implementation-defined behavior.

• The locations within #pragma directives where header name preprocessing tokens are recognized (C11 6.4, C11 6.4.7).
• How sequences in both forms of header names are mapped to headers or external source file names (C90 6.1.7, C99 and C11 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 and C11 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 and C11 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 and C11 6.10.2).
• How the named source file is searched for in an included ‘""’ delimited header (C90 6.8.2, C99 and C11 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 and C11 6.10.2).
• The nesting limit for #include processing (C90 6.8.2, C99 and C11 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 and C11 6.10.3.2).
• The behavior on each recognized non-STDC #pragma directive (C90 6.8.6, C99 and C11 6.10.6).

  See Pragmas, 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, C11 6.10.8.1).