tidy-html5/README/API_AND_NAMESPACE.md

The LibTidy API and Namespacing

Introduction

If you're just getting started working with LibTidy, some of the design choices may seem overwhelming if you're not a seasoned C veteran. Hopefully this article will give a decent overview, encouraging you to explore and contribute to the LibTidy code.

This article will discuss briefly:

• How LibTidy achieves namespacing in C

• Explanations for some of the bizzarre, do-nothing macros.

• Opaque types

• How to add new functions to the LibTidy API.

Namespacing

The C language does not support built in namespacing, but it is subject to namespace collision, especially when a library is statically linked. LibTidy tries to get around this by making a compromise between human-readable names and making the names random enough to avoid a collision.

As you browse Tidy's code, you'll notice many uses of a macro function – TY_() – applied to the function names of non-static functions. The preprocessor thus resolves all of these function names to prvTidyFunction, thus ensuring a clear namespace and avoiding the possibility of collisions (unless some other library has thoughtlessly borrowed our prefix for the same). For example, TY_(getNextOptionPick) will resolve to prvTidygetNextOptionPick when compiled.

Of course, static functions are immune to the issue of namespace pollution, so in general you will really only use this technique for functions that must be accessible from outside of your new file, such as functions that you want to expose to the API.

Macros for documentation

TIDY_EXPORT and TIDY_CALL are defined to be NULL, i.e., when compiled they resolve to nothing. These are used exclusively for documenting functions that are part of the API defined in tidy.h and the implementation in tidylib.c. For example, in tidy.h:

TIDY_EXPORT TidyIterator TIDY_CALL getWindowsLanguageList();

The TIDY_EXPORT call clearly indicates that this function prototype is meant to be exported from the API, and TIDY_CALL clearly indicates that the function is called from within LibTidy.

Although this makes things obvious from the documentation perspective, the truth is a little murkier. In some environments one might define TIDY_EXPORT and TIDY_CALL differently in order to control compiler behavior, especially in environments that have special requirements for dynamic libraries. In general, though, you shouldn't have to worry about this.

External types are opaque

In several spots the source code indicates that a particular structure is "opaque." This simply means that API users cannot see inside of them, and they have to depend on accessor functions to gain access to the sweet fruit that is within. This is a design choice that makes LibTidy highly portable and makes it accessible to multitudes of other languages that can communicate with a C API.

Take tidyDoc for example, as it's the most fundamental datatype within LibTidy. As an API user, you can have a reference to a tidyDoc, and you're going to pass it around a lot to accessor functions (such as tidyCleanAndRepair), and you know that it contains lots of good stuff, but you're not allowed to peek inside of it unless an accessor function is provided. Think of it as a token that you pass around, and nothing more.

Internally, the type is cast to a native C structure of type tidyDocImpl, and so if you decide to become a Tidy developer, you have the choice to access the item fully.

If you extend Tidy's API, it's important to respect this design choice, even if only writing functionality for the console application (which is, of course, simply an implementor of LibTidy).

How to add new functions to LibTidy

All of the information above is useful for anyone who wants to browse Tidy's source code, or use the API, or understand Tidy better, but it all comes together nicely when you want to extend the API. This quick lesson will show you how to do so, using tidyLocalizedString() as an example.

Behind the scenes

The first thing we need to do is have the internal version of the function that we want to add. Tidy has module that handles localization: language.h/c. In the header is where we define the interface to LibTidy, which should be namespaced according to the discussion above. We can declare:

ctmbstr TY_(tidyLocalizedString)( uint messageType );

…and of course implement it in the .c file.

Now you have a decision to make: if you plan to use this function internally, you're going to have to import the header into other modules that require the function. This can lead to painful compile-time consequences. However since we want to expose this particular function to the API, it will be visible within TidyLib, so we can use the public API internally, too.

The API

Once implemented, we want a pretty, public-facing name for our tidyLocalizedString() function, which appropriate is tidyLocalizedString(). Add the declaration to tidy.h:

TIDY_EXPORT ctmbstr TIDY_CALL tidyLocalizedString( uint messageType );

…and now the publicly exposed interface knows that your function exists. All that's left to do is add the language.h header to tidylib.c, and then implement it there:

ctmbstr TIDY_CALL tidyLocalizedString( uint messageType )
{
    return TY_(tidyLocalizedString)( messageType );
}

Congratulations, you can now expose new functionality to the API.

API functions for opaque types

For a more complicated example that demonstrates how to use opaque types (and also the TidyIterator type) have a look at the implementation of getWindowsLanguageList(), and its partners *getNextWindowsLanguage(), TidyLangWindowsName(), and TidyLangPosixName(). These demonstrate how to:

• implement iteration for structures with multiple records.
• write a function in tidylib.c that converts between the exposed, opaque type and the internal, implementation type.
• further reinforce how functionality is added to the API.