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+.. _preprocessor:
+
+=================
+Text Preprocessor
+=================
+
+The build system contains a text preprocessor similar to the C preprocessor,
+meant for processing files which have no built-in preprocessor such as XUL
+and JavaScript documents. It is implemented at ``python/mozbuild/mozbuild/preprocessor.py`` and
+is typically invoked via :ref:`jar_manifests`.
+
+While used to preprocess CSS files, the directives are changed to begin with
+``%`` instead of ``#`` to avoid conflict of the id selectors.
+
+Directives
+==========
+
+Variable Definition
+-------------------
+
+define
+^^^^^^
+
+::
+
+   #define variable
+   #define variable value
+
+Defines a preprocessor variable.
+
+Note that, unlike the C preprocessor, instances of this variable later in the
+source are not automatically replaced (see #filter). If value is not supplied,
+it defaults to ``1``.
+
+Note that whitespace is significant, so ``"#define foo one"`` and
+``"#define foo one "`` is different (in the second case, ``foo`` is defined to
+be a four-character string).
+
+undef
+^^^^^
+
+::
+
+   #undef variable
+
+Undefines a preprocessor variable.
+
+Conditionals
+------------
+
+if
+^^
+
+::
+
+   #if variable
+   #if !variable
+   #if variable==string
+   #if variable!=string
+
+Disables output if the conditional is false. This can be nested to arbitrary
+depths. Note that in the equality checks, the variable must come first, and
+the comparison operator must not be surrounded by any whitespace.
+
+else
+^^^^
+
+::
+
+   #else
+
+Reverses the state of the previous conditional block; for example, if the
+last ``#if`` was true (output was enabled), an ``#else`` makes it off
+(output gets disabled).
+
+.. warning:: An ``#else`` is relative to the last conditional block only,
+   unlike the C preprocessor.
+
+   It does not matter whether any blocks before it were true. This behavior
+   changed on trunk (Gecko 1.9) on 2006-12-07; see Bug 277122 for details.
+
+::
+
+   #if 1
+     always included
+   #elif 1
+     never included
+   #else
+     always included
+   #endif
+
+endif
+^^^^^
+
+::
+
+   #endif
+
+Ends the conditional block.
+
+ifdef / ifndef
+^^^^^^^^^^^^^^
+
+::
+
+   #ifdef variable
+   #ifndef variable
+
+An ``#if`` conditional that is true only if the preprocessor variable
+variable is defined (in the case of ``ifdef``) or not defined (``ifndef``).
+
+elif / elifdef / elifndef
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+   #elif variable
+   #elif !variable
+   #elif variable == string
+   #elif variable != string
+   #elifdef variable
+   #elifndef variable
+
+A shorthand to mean an ``#else`` combined with the relevant conditional.
+The following two blocks are equivalent::
+
+   #ifdef foo
+     block 1
+   #elifdef bar
+     block 2
+   #endif
+
+::
+
+   #ifdef foo
+     block 1
+   #else
+   #ifdef bar
+     block 2
+   #endif
+   #endif
+
+.. warning:: An ``#elif``, ``#elifdef``, or ``#elifndef`` is relative to
+   the last conditional block only (as well as the condition it implies),
+   unlike the C preprocessor. It does not matter whether any blocks before
+   it were true. This behavior changed on trunk (Gecko 1.9) on 2006-12-07.
+   See Bug 277122 for details.
+
+File Inclusion
+--------------
+
+include
+^^^^^^^
+
+::
+
+   #include filename
+
+The file specified by filename is processed as if the contents was placed
+at this position. This also means that preprocessor conditionals can even
+be started in one file and ended in another (but is highly discouraged).
+There is no limit on depth of inclusion, or repeated inclusion of the same
+file, or self inclusion; thus, care should be taken to avoid infinite loops.
+
+includesubst
+^^^^^^^^^^^^
+
+::
+
+   #includesubst @variable@filename
+
+Same as a ``#include`` except that all instances of variable in the included
+file is also expanded as in ``#filter`` substitution
+
+expand
+^^^^^^
+
+::
+
+   #expand string
+
+All variables wrapped in ``__`` are replaced with their value, for this line
+only. If the variable is not defined, it expands to an empty string. For
+example, if ``foo`` has the value ``bar``, and ``baz`` is not defined, then::
+
+   #expand This <__foo__> <__baz__> gets expanded
+
+Is expanded to::
+
+   This <bar> <> gets expanded
+
+filter / unfilter
+^^^^^^^^^^^^^^^^^
+
+::
+
+   #filter filter1 filter2 ... filterN
+   #unfilter filter1 filter2 ... filterN
+
+``#filter`` turns on the given filter.
+
+Filters are run in alphabetical order on a per-line basis.
+
+``#unfilter`` turns off the given filter. Available filters are:
+
+emptyLines
+   strips blank lines from the output
+slashslash
+   strips everything from the first two consecutive slash (``/``)
+   characters until the end of the line
+spaces
+   collapses consecutive sequences of spaces into a single space,
+   and strips leading and trailing spaces
+substitution
+   all variables wrapped in @ are replaced with their value. If the
+   variable is not defined, it is a fatal error. Similar to ``#expand``
+   and ``#filter``
+attemptSubstitution
+   all variables wrapped in ``@`` are replaced with their value, or an
+   empty string if the variable is not defined. Similar to ``#expand``.
+
+literal
+^^^^^^^
+
+::
+
+   #literal string
+
+Output the string (i.e. the rest of the line) literally, with no other fixups.
+This is useful to output lines starting with ``#``, or to temporarily
+disable filters.
+
+Other
+-----
+
+#error
+^^^^^^
+
+::
+
+   #error string
+
+Cause a fatal error at this point, with the error message being the
+given string.