Cap’n Proto comes with a command-line tool called
capnp intended to aid development and
debugging. This tool can be used to:
- Compile Cap’n Proto schemas to produce source code in multiple languages.
- Generate unique type IDs.
- Decode Cap’n Proto messages to human-readable text.
- Encode text representations of Cap’n Proto messages to binary.
- Evaluate and extract constants defined in Cap’n Proto schemas.
This page summarizes the functionality. A complete reference on the command’s usage can be found by typing:
capnp compile -oc++ myschema.capnp
This generates files
myschema.capnp.c++ which contain C++ source code
corresponding to the types defined in
myschema.capnp. Options exist to control output location
and import paths.
The above example generates C++ code, but the tool is able to generate output in any language
for which a plugin is available. Compiler plugins are just regular programs named
capnpc-language. For example, the above command runs
capnpc-c++. More on how to write
Note that some Cap’n Proto implementations (especially for interpreted languages) do not require generating source code.
capnp decode myschema.capnp MyType < message.bin > message.txt
capnp decode reads a binary Cap’n Proto message from standard input and decodes it to a
human-readable text format (specifically, the format used for specifying constants and default
values in the schema language). By default it
expects an unpacked message, but you can decode a
packed message with the
capnp encode myschema.capnp MyType < message.txt > message.bin
capnp encode is the opposite of
capnp decode: it takes a text-format message on stdin and
encodes it to binary (possibly packed,
This is mainly useful for debugging purposes, to build test data or to apply tweaks to data
capnp decode. You should not rely on
capnp encode for encoding data written
and maintained in text format long-term – instead, use
capnp eval, which is much more powerful.
capnp eval myschema.capnp myConstant
This prints the value of
myConstant, a const declaration, after
applying variable substitution. It can also output the value in binary format (
At first glance, this may seem no more interesting that
capnp encode: the syntax used to define
constants in schema files is the same as the format accepted by
capnp encode, right? There is,
however, a big difference: constants in schema files may be defined in terms of other constants,
which may even be imported from other files.
As a result,
capnp eval is a great basis for implementing config files. For example, a large
company might maintain a production server that serves dozens of clients and needs configuration
information about each one. Rather than maintaining the config as one enormous file, it can be
written as several separate files with a master file that imports the rest.
Such a configuration should be compiled to binary format using
capnp eval before deployment,
in order to verify that there are no errors and to make deployment easier and faster. While you
could technically ship the text configs to production and have the servers parse them directly
capnp::SchemaParser), encoding before deployment is more efficient and robust.