Capnp.jl - Julia plugin for Cap'n'proto

This is currently a prototype.

Install & use

Install from JuliaHub:

] add Capnp

Download capnpc-jl from this repository and generate code for a schema with

capnpc -o./capnpc-jl example/addressbook.capnp


See the addressbook example in the example directory.

Generated names

Capnp.jl supports namespace annotations and translates them into Julia modules. E.g. using $Cxx.namespace("capnp::schema"); generates code in module capnp.schema. Note Julia modules can't reference each other in a cycle.

To start reading a reader has to be opened with a stream, e.g. message = Capnp.MessageReader(stdin) and root reader created, e.g. for a struct A reader = root_A(message). To write, a writer has to be created, e.g. message = Capnp.AllocMessageBuilder(), root writer initialised, e.g. writer = initRoot_A(message), and finally the message should be written to a stream writeMessageToStream(message, stdout).

For a struct A with a field xy function A_getXy(reader) is generated. If A has a nested struct B with a field yz then A_B_getYz() is generated.

For reading and writing lists you can use brackets [] but note this is 1-based as is usual in Julia. For writing lists you need to initialise them with A_initBs(a_writer, number_of_items).

If struct A has a union group u then A_u_union enum is generated as well as function A_u_which(a_reader). For an anonymous union the names would be A_union and A_which. Sett union slot xy with A_u_setXy(a_writer, value) or A_u_setXy(a_writer) if xy is Void. For xy of struct type use xy_writer = A_u_initXy(a_writer).

(If you feel there's something missing in this description then please let me know.)


See src/Capnp.jl for description of code structure. To generate the code for the Capnp schema use capnpc -o./capnpc-jl src/schema.capnp.

Some things to work on:

  • Add a test like the addressbook integration test but one testing the capnp schema reading/writing.
  • Emit types that make user code safer.
  • Zero allocations when reading or writing using a big enough buffer. Start by adding a test.
  • Support capnp's packing.
  • Generate smaller code.
  • Support generics.
  • Test in some production environment. (Please let me know if you do.)
  • Initialisation/default values.

For debugging it can be useful to save a message to a file and use xxd --bits --cols 8. How to Write Compiler Plugins has other good tips, especially the bit with printing annotated schema (capnp compile -ocapnp schema.capnp).

Finally, run tests with julia --project test/runtests.jl and format code (using JuliaFormatter; format("."), except generated code; git checkout src/schema.capnp.jl).