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### 4.13 Preprocessing directives

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).