## Libraries

C doesn't need name mangling, and there's a lot of C libraries, so it's commonly used as a target for FFIs, and one can rather easily get bindings to a C library from a few more languages with SWIG. Along with being portable, fast, lightweight, widely known, and having the unixy infrastructures aimed at it, it makes C the primary language choice for widely reusable libraries.

Some of the other options that allow library calls from other common languages:

- C++, with `extern "C"`
- Haskell, with `foreign export`
- Rust, apparently

In practice it seems to be the most reliable approach, and one with least overhead.

Functions from scripting languages can also be called from C, but not including those in this section.

## File formats and protocols

... Including all kinds of services and conventions. Even interaction with libraries (calling conventions, data types) can be seen as a protocol. An issue with those is that the most commonly used ones (not counting library calls here) tend to be controversial and awkward, with numerous competing alternatives.

Textual/unixy formats/protocols also tend to get clunky, but at least they are usable for human-computer interaction.

File formats are needed for storage, network protocols – for networking, but the practice of their use in place of regular function calls doesn't seem to be great. Maybe it has something to do with the ease of introducing new ones, and the complexity of everyone settling on one to use.

Another issue is that setting up the environments for those is non-trivial: once again, the existing infrastructure doesn't facilitate use of arbitrary user daemons, and running the programs that provide services accessible via some kind of RPC as child processes would be both cumbersome and awkward.

There may be a cross of this with the previous section, using message-oriented middleware, but then you get awkward bits of both at once.

## Duct tape

Duct tape programming is always there, and used (among other things) to glue things together – especially when it's hard/impossible to get a proper interface. It may involve ad hoc protocols (say, one program writes into a file or a database, and another one reads from it then), or actually using those competing protocols when there's no choice: even if one doesn't like them, they are usually better than unspecified ad hoc ones, which are used quite often as well (in fact a part of my job is reverse engineering of those, and I'd take any specified protocol over those. Well, almost any: there are rather awkward specified ones too).

Child processes, or interpreters-as-libraries, may be counted here too.

But usually it's a collection of dirty hacks that virtually nobody likes or would choose if there was a choice: if there is useful code that is only accessible via some unusual or awkward API/protocol, chances are it'd give rise to reimplementations in more languages.

## Summary

There are ways to achieve interoperability and code reuse, even too many of those. Yet it seems that it gets achieved any reliably, and with the least amount of controversy, just when a system/infrastructure supports it: it becomes a de facto standard, and languages have to have good support for that – as it is with C FFI and text streams (stdin/stdout/stderr, sockets, or other standard IPC facilities). So widely reusable code has to be easily available over one of those.

Although those also tend to be inflexible, and there is motivation to get implementations native to a given language still. But that is easier to postpone (and to focus on tasks at hand) when practically reusable libraries are available.