package capnp

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OCaml code generation plugin for the Cap'n Proto serialization framework


Dune Dependency






  • Update README to talk about stdint, not uint (reported by @liyishuai).
    Also, remove out-of-date list of runtime packages needed. The build system can sort that out.

  • Remove all inlined attributes (@talex5 #87 #88).
    These cause confusing compiler warnings for users, which can't be disabled automatically, and the inlining isn't being used anyway since

  • Fix "Unknown interface" error (@talex5 #85).
    It reported the UUID of the actual object, not the UUID that was requested. Also, add Registry.pp_interface for better error messages.

  • Minor opam fixes (@talex5 #84).
    Depend on dune >= 2.3 and don't try to build benchmarks; that only works on some platforms.

  • Bump minimum OCaml version to 4.08.
    4.07 doesn't work due to


New features:

  • Add Codecs.serialize_{fold,iter}_copyless (@Cjen1, #71). Avoids a copy when writing out fragments.


  • Add example of how to use the library (@rottened23, #62).

  • Extract LICENSE and identify as BSD-2-Clause (@tmcgilchrist, #80).

Build system:

  • Remove compiler flags for flambda (@talex5, #83). This was very slow to compile on OCaml 4.14, and the performance benefit is minimal.

  • Update to dune 2 (@talex5, #77).

  • Dune should not be a build dependency (@talex5, #67).

  • Replace Uint32.of_int in unit-tests with of_string (@talex5, #73). Fixed tests on 32-bit systems.

  • Remove Travis CI (@talex5, #74). Replaced by ocaml-ci.

  • Switch from "capnpc" to "capnp compile" (@talex5, #75).

  • Benchmarks: remove incorrect test for 64-bit on Windows (@dra27, #72).


  • Upgrade from uint to stdint (@talex5, #64). From uint 2.0.1, these are compatible. Projects using the schema compiler should either upgrade to stdint themselves, or add a lower bound of uint >= 2.0.1. Otherwise, the build may fail because of a missing dependency on stdint.

  • Require base >= 0.11 for Base.Queue (@talex5, #61).

  • Fix compilation on OCaml 4.08 (@talex5, #63).

  • Fix generated files when one interface imports another (@talex5, #65, reported by TealG).


This release is mostly about reducing the number of dependencies. The only API change is that write_message_to_file_robust has gone.

  • Remove extunix dependency (#57). write_message_to_file_robust was its only user, and wasn't used by anything. Using fsync to ensure a file is written to disk is a general function that should be handled by the user of capnp where needed. Removing extunix also removes the indirect dependency on camlp4, which in turn allows capnp to build with OCaml 4.08 (and to build faster on all versions).

  • Remove Pervasives qualifier (#57). This is needed to support OCaml 4.08 without warnings.

  • Remove dependency on Core from benchmarks (#55). This was the only remaining use of the library.

  • Replace uses of Core_kernel with plain Base and Stdio in the compiler (#56) and tests (#59). This greatly reduces the number of libraries you need to install to install capnp.

  • Update tests for the current quickcheck API (#54).

  • Switch from jbuilder to dune (#54).

  • Require OCaml >= 4.03 (#54). This allows us to drop some complexity from the jbuilder files, allowing them to be upgraded automatically by dune.

  • Upgrade to opam 2 format (#54).

  • Convert the changelog to markdown (#58). This allows it to be used with dune-release.

With these changes, the following 49 libraries that were needed to install capnp-ocaml 3.2.1 are no longer required: base_bigstring, base_quickcheck, bin_prot, camlp4, core_kernel, extunix, fieldslib, jane-street-headers, jst-config, num, ocaml-compiler-libs, ocaml-migrate-parsetree, octavius, parsexp, ppx_assert, ppx_base, ppx_bench, ppx_bin_prot, ppx_compare, ppx_custom_printf, ppx_derivers, ppx_enumerate, ppx_expect, ppx_fail, ppx_fields_conv, ppx_hash, ppx_here, ppx_inline_test, ppx_jane, ppx_js_style, ppx_let, ppxlib, ppx_module_timer, ppx_optcomp, ppx_optional, ppx_pipebang, ppx_sexp_conv, ppx_sexp_message, ppx_sexp_value, ppx_stable, ppx_typerep_conv, ppx_variants_conv, re, seq, sexplib, splittable_random, time_now, typerep and variantslib.


Other changes

  • Update for API changes in Base and Core_kernel (#52).


Backwards-incompatible changes

  • The capnp opam package no longer depends on the C++ compiler (#47). If your project compiles schema files, you should add "conf-capnproto" {build} to your opam dependencies.

Other changes

  • Add support for OCaml 4.06 (#45).

  • Update for API changes in Base and Core_kernel (#44).


Backwards-incompatible changes

  • The Unix-specific IO module has been moved from Capnp.IO to Capnp_unix.IO, and is now part of the new capnp.unix ocamlfind library. Both capnp and capnp.unix ocamlfind libraries are provided by the capnp opam package.

New features

  • The core capnp library no longer depends on Core_kernel or Unix. This makes binaries using the library several MB smaller and allows capnp to be used in Mirage unikernels.


Backwards-incompatible changes

  • The Builder.X.reader_t and Reader.X.builder_t types have gone (to avoid confusion with the generic reader_t and builder_t types). Use X.struct_t reader_t and X.struct_t builder_t instead. Builder.X.t is still defined (as an alias for t builder_t), and similarly for Reader.X.t.

  • StructStorage.t now takes an extra type parameter. This is a phantom type, used to indicate what kind of struct it represents. This extra parameter has also been added to Object.t.

  • In the generated files, the unique types names (e.g. reader_t_Foo_14133145859926553711) have gone. If you need to refer to these directly for some reason, use the replacement polymorphic type instead (e.g. Foo_c42305476bb4746f reader_t).

New features

  • RPC support (#35 and #36). The compiler now generates readers and builders for parameters and results, interface and method IDs, and a dispatch method for services. There is a new MakeRPC functor in the generated files that takes an extended version of MessageSig. The original Make is still provided - it is now a convenience wrapper around MakeRPC that passes some dummy functions. An RPC provider is available at

  • Capability references can now be attached to messages. The getters and setters that previously took Uint32.t indexes can now take capabilities, as defined by an RPC provider library. The default RPC.None provider exposes the raw descriptor index, as before.

  • Better AnyPointer support (#11). The new of_pointer and init_pointer functions can be used to read and add structs of any type to AnyPointer fields. The new pointer_set_reader can be used to set a pointer by copying from a reader.

  • Replace camlp4 includes with flambda inlining (#23). The files that were previously copied into each generated file are now regular modules in the library, making them available to other code too. This makes the generated files much smaller. There is no performance penalty (when using flambda).

  • Port to jbuilder (#24). Also, the required schema files are now included in the repository rather than being downloaded by the build scripts, which was fragile.

  • Add support for cycles (#29). Before, the compiler would abort if a schema file contained cycles, but this is very common (especially with interfaces). It now generates forward references where needed.

  • Add parameterised reader_t and builder_t types (#32 and #33). Instead of providing Reader.Foo.t and Builder.Foo.t as abstract types, provide a single Foo.struct_t and use struct_t reader_t and struct_t builder_t for its reader and builder. This makes it possible to define generic functions that work on all readers or builders, without having to generate a separate function for each one. The types are shared between files, so you can write generic code to work with schema files you haven't seen (needed for the RPC system).

  • Use polymorphic variants for phantom types (#34). Instead of declaring a load of unique type names at the start of the file, use polymorphic variants. These don't need to be declared before use. The node ID is now formatted as hex, to make it shorter and to match the format used in the schema files. There are new generic reader_of_builder and message_of_builder functions in StructStorage.

Other changes

  • All uses of Obj.magic have been removed from the generated code.

  • Update to latest core_kernel (#26). Avoids conflict with newer sexplib.

  • Fix some compiler warnings (#28). Jbuilder turns on more warnings by default.

  • Add missing test dependency on ounit (#30).

  • Fix code generation bug in 2.1.1 (#10). A missing space prevented the generated files from compiling. Added Travis CI tests to detect this kind of thing automatically.


  • Fix Not_found exception when a capnp import is not referenced within the schema file.

  • Omit references to capnp imports which are not used within the generated code. In particular, importing "/capnp/c++.capnp" no longer generates a reference to an OCaml module with the confusing name of C2b2b, and there should no longer be a need to separately compile that imported schema file.


  • Depend on Core_kernel rather than Core. Thanks to Matthew Maurer for driving this change.

  • Fix compatibility issue when building against Core 112.35.00.


  • Invoke OCaml scripts using -I $OCAML_TOPLEVEL_PATH, to avoid missing topfind.


Backwards-incompatible changes

  • Module Codecs: change API to accept the more natural BytesMessage.Message.t instead of Bytes.t list.

  • Module Codecs: change API to accept a compression specifier, instead of using separate functions and separate types for compressed and uncompressed streams.

  • Module Message: to_storage now returns a list of (storage, length) pairs.

Other changes

  • Module Codecs: fix incorrect encoding of framing headers (for example, as generated by serialize).

  • Module Codecs: fix infinite loop in PackedStream decoding.

  • Module Codecs: reduce serialized message sizes, by omitting unused allocation regions from the serialized messages.

  • Instantiate BytesMessage = Message.Make(BytesStorage), so the user doesn't always need to. (Implementation is furthermore defunctorized for improved performance.)

  • New module IO: functions for moving message data through various types of I/O channels.

  • Compiler now says something about files it created.

  • Corrected compilation errors when using 4.02 -safe-string.

  • Significant performance improvements across the board.


  • Avoid use of GNU install features in omake install. This corrects installation problems on OS X and (most likely) other BSDs.


  • Initial release.