..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. _preprocessing-directives-implementation:

Preprocessing Directives
************************

See :ref:`cpp: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 :samp:`<>` 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 :samp:`""`
  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 :samp:`#` operator inserts a :samp:`\\` character before
  the :samp:`\\` 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 :ref:`cpp:pragmas`, for details of
  pragmas accepted by GCC on all targets.  See :ref:`pragmas`, 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).