Tutorial

Build

$ pixi install
$ pixi run configure
$ pixi run build
$ pixi run test

Bare Meson: meson setup build && meson compile -C build && meson test -C build.

Janet: demo buffer

The in-tree demo builder writes a struct with a UInt32, a Text, and a List(Text). After (import capnp) (or after the host calls capnp_janet_register):

(import capnp)

(def buf (capnp/build-demo 42 "hello" ["a" "bb"]))
(def msg (capnp/message-from-buffer buf))
(def root (capnp/root msg))
(print (capnp/get-u32 root 0))        # 42
(print (capnp/get-text root 0))       # hello
(def items (capnp/getp root 1))
(print (capnp/list-get-text items 1)) # bb

message-from-buffer copies so the Janet buffer may be reused. message-view-buffer aliases the caller’s buffer and keeps it GC-marked for the view lifetime.

AddressBook

test/test_addressbook.c sets fields in schema order and memcmp=s =test/fixtures/addressbook_alice_bob.bin from official capnp encode. Regenerate goldens with ./scripts/gen-sample-fixtures.sh (needs capnp on PATH).

Schema: schema/addressbook.capnp (Alice / Bob, phones, employment union). Packed and canonical goldens sit next to the unpacked .bin.

C embed

#include <capnp-janet/capnp_message.h>
#include <capnp-janet/capnp_janet.h>

/* after janet_init(env): */
capnp_janet_register(env);

capnp_message_t msg;
capnp_message_view_flat(&msg, bytes, len);
capnp_ptr_t root;
capnp_root(&msg, &root);
uint32_t id = capnp_get_u32(&root, 0, 0);

Link libcapnp_janet (pkg-config: capnp-janet). Compiled .jimage packs cannot store the native capnp/* CFUNS; the host re-registers them, then unmarshals. See architecture.