This is protobuf-c, a C implementation of the Google Protocol Buffers
data serialization format.

It includes libprotobuf-c, a pure C library that implements protobuf
encoding and decoding, and protoc-c, a code generator that converts
Protocol Buffer .proto files to C descriptor code, based on the
original protoc.

protobuf-c formerly included an RPC implementation; that code has been
split out into the protobuf-c-rpc project.
