capnpc-c
========

This is a C plugin for [Cap'n Proto](http://kentonv.github.io/capnproto), an
efficient protocol for sharing data and capabilities.

> ## Security warning!

> The generated code assumes all input to be trusted. Do NOT use with
> untrusted input! There is currently no code in place to check if
> structures/pointers are within bounds.

This is only the code generator plugin, to properly make use of it you
need to download, build and install capnpc and then build and install
this project and then you can utilize it as:

```sh
capnpc compiler/test.capnp -oc
```

[![Build Status](https://travis-ci.org/opensourcerouting/c-capnproto.svg?branch=master)](https://travis-ci.org/opensourcerouting/c-capnproto)

## Building on Linux

```sh
git clone --recurse-submodules https://github.com/opensourcerouting/c-capnproto
cd c-capnproto
autoreconf -f -i -s
./configure
make
make check
```

## Building with Meson

```sh
git clone --recurse-submodules https://github.com/opensourcerouting/c-capnproto
cd c-capnproto
meson setup build
meson compile -C build
build/capn-test
```

## Usage

### Generating C code from a `.capnp` schema file

The `compiler` directory contains the C language plugin (`capnpc-c`) for use with the `capnp` tool: https://capnproto.org/capnp-tool.html.

`capnp` will by default search `$PATH` for `capnpc-c` - if it's on your PATH, you can generate code for your schema as follows:

```sh
capnp compile -o c myschema.capnp
```

Otherwise, you can specify the path to the c plugin:

```sh
capnp compile -o ./capnpc-c myschema.capnp
```

`capnp` generates a C struct that corresponds to each capn proto struct, along with read/write functions that convert to/from capn proto form.

If you want accessor functions for struct members, use attribute  `fieldgetset` in your `.capnp` file as follows:

```capnp
using C = import "${c-capnproto}/compiler/c.capnp";

$C.fieldgetset;

struct MyStruct {}
```

### Example C code

See the unit tests in [`tests/example-test.cpp`](tests/example-test.cpp).
The example schema file is [`tests/addressbook.capnp`](tests/addressbook.capnp).
The tests are written in C++, but only use C features.

You need to compile these runtime library files and link them into your own project's binaries:

* [`lib/capn.c`](lib/capn.c)
* [`lib/capn-malloc.c`](lib/capn-malloc.c)
* [`lib/capn-stream.c`](lib/capn-stream.c)

Your include path must contain the runtime library directory
[`lib`](lib). Header file [`lib/capnp_c.h`](lib/capnp_c.h) contains
the public interfaces of the library.

Using make-based builds, make may try to compile `${x}.capnp` from
`${x}.capnp.c` using its built-in rule for compiling `${y}` from
`${y}.c`. You can either disable make's built-in compile rules or just
this specific case with the no-op rule: `%.capnp: ;`.

For further reference, please see the other unit tests in [`tests`](tests), and header file [`lib/capnp_c.h`](lib/capnp_c.h).

The project [`quagga-capnproto`](https://github.com/opensourcerouting/quagga-capnproto) uses `c-capnproto` and contains some good examples, as found with [this github repository search](https://github.com/opensourcerouting/quagga-capnproto/search?utf8=%E2%9C%93&q=capn&type=):

* Serialization in function [`bgp_notify_send()`](https://github.com/opensourcerouting/quagga-capnproto/blob/27061648f3418fac0d217b16a46add534343e841/bgpd/bgp_zmq.c#L81-L96) in file `quagga-capnproto/bgpd/bgp_zmq.c`
* Deserialization in function [`qzc_callback()`](https://github.com/opensourcerouting/quagga-capnproto/blob/27061648f3418fac0d217b16a46add534343e841/lib/qzc.c#L249-L257) in file `quagga-capnproto/lib/qzc.c`

## Status

This is a merge of 3 forks of [James McKaskill's great
work](https://github.com/jmckaskill/c-capnproto), which has been untouched for
a while:

- [liamstask's fork](https://github.com/liamstask/c-capnproto)
- [baruch's fork](https://github.com/baruch/c-capnproto)
- [kylemanna's fork](https://github.com/kylemanna/c-capnproto)