preprocessor.rst - mozsearch

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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.

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).

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

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

dumbComments

    dumbComments: empties out any line that consists of optional whitespace

    followed by a ``//``. Good for getting rid of comments that are on their

    own lines, and being smarter with a simple regexp filter is impossible

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.