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diff --git a/build/docs/preprocessor.rst b/build/docs/preprocessor.rst new file mode 100644 index 0000000000..5ce9092ed9 --- /dev/null +++ b/build/docs/preprocessor.rst @@ -0,0 +1,219 @@ +.. _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. |