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

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 for 32-bit x86. While theoretically slower, at least all compilers can access packed records afaik, and the official alignment requirements have changed over time as the architecture evolved
• 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.
• Do not return structured types like records/structs or arrays as returntype of functions.
• 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.
• Always pair a callbacks with an user configurable context. This can be used by the user to give the the callback state without defining global variables. (like e.g. an object instance)
• 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.
• don't use C style varargs parameters. Not all languages allow variable number of parameters in an unsafe way (*) 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.

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.

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).