How to design a C / C++ library to be usable in many client languages?

Question

I'm planning to code a library that should be usable by a large number of people in on a wide spectrum of platforms. What do I have to consider to design it right? To make this questions more specific, there are four "subquestions" at the end.

Choice of language

Considering all the known requirements and details, I concluded that a library written in C or C++ was the way to go. I think the primary usage of my library will be in programs written in C, C++ and Java SE, but I can also think of reasons to use it from Java ME, PHP, .NET, Objective C, Python, Ruby, bash scrips, etc... Maybe I cannot target all of them, but if it's possible, I'll do it.

Requirements

It would be to much to describe the full purpose of my library here, but there are some aspects that might be important to this question:

• The library itself will start out small, but definitely will grow to enormous complexity, so it is not an option to maintain several versions in parallel.
• Most of the complexity will be hidden inside the library, though
• The library will construct an object graph that is used heavily inside. Some clients of the library will only be interested in specific attributes of specific objects, while other clients must traverse the object graph in some way
• Clients may change the objects, and the library must be notified thereof
• The library may change the objects, and the client must be notified thereof, if it already has a handle to that object
• The library must be multi-threaded, because it will maintain network connections to several other hosts
• While some requests to the library may be handled synchronously, many of them will take too long and must be processed in the background, and notify the client on success (or failure)

Of course, answers are welcome no matter if they address my specific requirements, or if they answer the question in a general way that matters to a wider audience!

My assumptions, so far

So here are some of my assumptions and conclusions, which I gathered in the past months:

• Internally I can use whatever I want, e.g. C++ with operator overloading, multiple inheritance, template meta programming... as long as there is a portable compiler which handles it (think of gcc / g++)
• But my interface has to be a clean C interface that does not involve name mangling
• Also, I think my interface should only consist of functions, with basic/primitive data types (and maybe pointers) passed as parameters and return values
• If I use pointers, I think I should only use them to pass them back to the library, not to operate directly on the referenced memory
• For usage in a C++ application, I might also offer an object oriented interface (Which is also prone to name mangling, so the App must either use the same compiler, or include the library in source form)
• Is this also true for usage in C# ?
• For usage in Java SE / Java EE, the Java native interface (JNI) applies. I have some basic knowledge about it, but I should definitely double check it.
• Not all client languages handle multithreading well, so there should be a single thread talking to the client
• For usage on Java ME, there is no such thing as JNI, but I might go with Nested VM
• For usage in Bash scripts, there must be an executable with a command line interface
• For the other client languages, I have no idea
• For most client languages, it would be nice to have kind of an adapter interface written in that language. I think there are tools to automatically generate this for Java and some others
• For object oriented languages, it might be possible to create an object oriented adapter which hides the fact that the interface to the library is function based - but I don't know if its worth the effort

Possible subquestions

• is this possible with manageable effort, or is it just too much portability?
• are there any good books / websites about this kind of design criteria?
• are any of my assumptions wrong?
• which open source libraries are worth studying to learn from their design / interface / souce?
• meta: This question is rather long, do you see any way to split it into several smaller ones? (If you reply to this, do it as a comment, not as an answer)

Answer 1

I don't know but if it's for Windows then you might try either a straight C-like API (similar to the WINAPI), or packaging your code as a COM component: because I'd guess that programming languages might want to be able to invoke the Windows API, and/or use COM objects.

Answer 2

Mostly correct. Straight procedural interface is the best. (which is not entirely the same as C btw(**), but close enough)

I interface DLLs a lot(*), both open source and commercial, so here are some points that I remember from daily practice, note that these are more recommended areas to research, and not cardinal truths:

• Watch out for decoration and similar "minor" mangling schemes, specially if you use a MS compiler. Most notably the stdcall convention sometimes leads to decoration generation for VB's sake (decoration is stuff like @6 after the function symbol name)
• Not all compilers can actually layout all kinds of structures:
  • so avoid overusing unions.
  • avoid bitpacking
  • and preferably pack the records. While slower, at least all compilers can access packed records afaik
• On Windows use stdcall. This is the default for Windows DLLs. Avoid fastcall, it is not entirely standarized (specially how small records are passed)
• Some tips to make automated header translation easier:
  • macros are hard to autoconvert due to their untypeness. Avoid them, use functions
  • Define separate types for each pointer types, and don't use composite types (xtype **) in function declarations.
  • follow the "define before use" mantra as much as possible, this will avoid users that translate headers to rearrange them if their language in general requires defining before use, and makes it easier for one-pass parsers to translate them. Or if they need context info to auto translate.
• Don't expose more than necessary. Leave handle types opague if possible. It will only cause versioning troubles later.
• always have a version check function (easier to make a distinction).
• be careful with enums and boolean. Other languages might have slightly different assumptions. You can use them, but document well how they behave and how large they are. Also think ahead, and make sure that enums don't become larger if you add a few fields, break the interface. (e.g. on Delphi/pascal by default booleans are 0 or 1, and other values are undefined. There are special types for C-like booleans (byte,16-bit or 32-bit word size, though they were originally introduced for COM, not C interfacing))
• I prefer stringtypes that are pointer to char + length as separate field (COM also does this). Preferably not having to rely on zero terminated. This is not just because of security (overflow) reasons, but also because it is easier/cheaper to interface them to Delphi native types that way.
• Memory always create the API in a way that encourages a total separation of memory management. IOW don't assume anything about memory management. This means that all structures in your lib are allocated via your own memory manager, and if a function passes a struct to you, copy it instead of storing a pointer made with the "clients" memory management. Because you will sooner or later accidentally call free or realloc on it :-)
• (implementation language, not interface), be reluctant to change the coprocessor exception mask. Some languages change this as part of conforming to their standards floating point error(exception-)handling.
• be careful with the coprocessor status word. It might be changed by others and break your code, and if you change it, other code might stop working. The status word is generally not saved/restored as part of calling conventions. At least not in practice.

(*) Delphi programmer by day, a job that involves interfacing a lot of hardware and thus translating vendor SDK headers. By night Free Pascal developer, in charge of, among others, the Windows headers.

(**) This is because what "C" means binary is still dependant on the used C compiler, specially if there is no real universal system ABI. Think of stuff like:

• C adding an underscore prefix on some binary formats (a.out, Coff?)
• sometimes different C compilers have different opinions on what to do with small structures passed by value. Officially they shouldn't support it at all afaik, but most do.
• structure packing sometimes varies, as do details of calling conventions (like skipping integer registers or not if a parameter is registerable in a FPU register)

===== automated header conversions ====

While I don't know SWIG that well, I know and use some delphi specific header tools( h2pas, Darth/headconv etc).

However I never use them in fully automatic mode, since more often then not the output sucks. Comments change line or are stripped, and formatting is not retained.

I usually make a small script (in Pascal, but you can use anything with decent string support) that splits a header up, and then try a tool on relatively homogeneous parts (e.g. only structures, or only defines etc).

Then I check if I like the automated conversion output, and either use it, or try to make a specific converter myself. Since it is for a subset (like only structures) it is often way easier than making a complete header converter. Of course it depends a bit what my target is. (nice, readable headers or quick and dirty). At each step I might do a few substitutions (with sed or an editor).

The most complicated scheme I did for Winapi commctrl and ActiveX/comctl headers. There I combined IDL and the C header (IDL for the interfaces, which are a bunch of unparsable macros in C, the C header for the rest), and managed to get the macros typed for about 80% (by propogating the typecasts in sendmessage macros back to the macro declaration, with reasonable (wparam,lparam,lresult) defaults)

The semi automated way has the disadvantage that the order of declarations is different (e.g. first constants, then structures then function declarations), which sometimes makes maintenance a pain. I therefore always keep the original headers/sdk to compare with.

The Jedi winapi conversion project might have more info, they translated about half of the windows headers to Delphi, and thus have enormous experience.

Answer 3

Regarding automatic wrapper generation, consider using SWIG. For Java, it will do all the JNI work. Also, it is able to translate complex OO-C++-interfaces properly (provided you follow some basic guidelines, i.e. no nested classes, no over-use of templates, plus the ones mentioned by Marco van de Voort).

Answer 4

NestedVM I think is going to be slower than pure Java because of the array bounds checking on the int[][] that represents the MIPS virtual machine memory. It is such a good concept but might not perform well enough right now (until phone manufacturers add NestedVM support (if they do!), most stuff is going to be SLOW for now, n'est-ce pas)? Whilst it may be able to unpack JPEGs without error, speed is of no small concern! :)

Nothing else in what you've written sticks out, which isn't to say that it's right or wrong! The principles sound (mainly just listening to choice of words and language to be honest) like roughly standard best practice but I haven't thought through the details of everything you've said. As you said yourself, this really ought to be several questions. But of course doing this kind of thing is not automatically easy just because you're fixed on perhaps a slightly different architecture to the last code base you've worked on...! ;)

My thoughts:

All your comments on C interface compatibility sound sensible to me, pretty much best practice except you don't seem to properly address memory management policy - some sentences a bit ambiguous/vague/wrong-sounding. The design of the memory management will be to a large extent determined by the access patterns made in your application, rather than the functionality per se. I suiggest you study others' attempts at making portable interfaces like the standard ANSI C API, Unix API, Win32 API, Cocoa, J2SE, etc carefully.

If it was me, I'd write the library in a carefully chosen subset of the common elements of regular Java and Davlik virtual machine Java and also write my own custom parser that translates the code to C for platforms that support C, which would of course be most of them. I would suggest that if you restrict yourself to data types of various size ints, bools, Strings, Dictionaries and Arrays and make careful use of them that will help in cross-platform issues without affecting performance much most of the time.

Answer 5

Hi, your assumptions seem ok, but i see trouble ahead, much of which you have already spotted in your assumptions. As you said, you can't really export c++ classes and methods, you will need to provide a function based c interface. What ever facade you build around that, it will remain a function based interface at heart.

The basic problem i see with that is that people choose a specific language and its runtime because their way of thinking (functional or object oriented) or the problem they address (web programming, database,...) corresponds to that language in some way or other. A library implemented in c will probably never feel like the libraries they are used to, unless they program in c themselves. Personally, I would always prefer a library that "feels like python" when I use python, and one that feels like java when I do Java EE, even though I know c and c++.

So your effort might be of little actual use (other than your gain in experience), because people will probably want to stick with their mindset, and rather re-implement the functionality than use a library that does the job, but does not fit.

I also fear the desired portability will seriously hamper development. Just think of the infinite build settings needed, and tests for that. I have worked on a project that tried to maintain compatibility for 5 operating systems (all posix-like, but still) and about 10 compilers, the builds were a nightmare to test and maintain.

Answer 6

Think C, nothing else. C is one of the most popular programming languages. It is widely used on many different software platforms, and there are few computer architectures for which a C compiler does not exist. All popular high-level languages provide an interface to C. That makes your library accessible from almost all platforms in existence. Don't worry too much about providing an Object Oriented interface. Once you have the library done in C, OOP, functional or any other style interface can be created in appropriate client languages. No other systems programming language will give you C's flexibility and potability.

Answer 7

Give it an XML interface, whether passed as a parameter and return value or as files through a command-line invocation. This may not seem as direct as a normal function interface, but is the most practical way to access an executable from, e.g., Java.