Eliminating Copies in C-FFI

We've run into a situation fairly regularly when wrapping C libraries where we'd like to avoid making extra copies of data when passing it between Dylan and C code via the C-FFI. Related to this, we want to be able to use multiple types from the Dylan side of things such as <buffer>, <byte-vector> or <byte-string> without having to have separate code paths for each of them. Each of these classes can store raw byte data that we may want to share with other code.

An example that we'll work with in this discussion comes from bindings (in simplified and excerpted forms) for the LevelDB library:

extern void leveldb_put(
    leveldb_t* db,
    const leveldb_writeoptions_t* options,
    const char* key, size_t keylen,
    const char* val, size_t vallen,
    char** errptr);


The easiest way to bind leveldb_put would be to use <C-string>:

define C-function %leveldb-put
  input parameter db_ :: <leveldb-t*>;
  input parameter options_ :: <leveldb-writeoptions-t*>;
  input parameter key_ :: <C-string>;
  input parameter keylen_ :: <size-t>;
  input parameter val_ :: <C-string>;
  input parameter vallen_ :: <size-t>;
  output parameter errptr_ :: <char**>;
  c-name: "leveldb_put";

However, this would mean that every key and value would have to be copied from an existing byte container (like <buffer>, <string> or <byte-vector>) to ...

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Binding nanomsg with melange

For those who haven't heard about it, nanomsg is a great library from Martin Sustrik. It provides high performance messaging with a simple API similar to that of BSD sockets. It bears some similarity to what someone might do with ZeroMQ and is licensed under the liberal terms of the MIT license. The nanomsg library is currently in a pre-alpha state.

I have recently written bindings for it for use with Dylan which are available as nanomsg-dylan.

In Dylan, we have a couple of options when writing bindings. We have a low level direct-c-ffi and a higher level C-FFI. Using the lower level interface is fairly tedious and verbose while using C-FFI is fairly convenient. But writing a binding using either involves a lot of work and hand-translation of the C APIs into the right Dylan definitions. (Currently, the direct-c-ffi system is not documented.)

This is where the melange tool is very useful. Melange can parse C headers and automatically generate the C-FFI bindings. While doing this code generation, it also handles details like automatically translations the C names into the correct Dylan names following the Dylan conventions.

Melange now has some preliminary documentation.

Using Melange

Melange generates a ...

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