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# μpb: small, fast C protos
μpb (often written 'upb') is a small
[protobuf](https://github.com/protocolbuffers/protobuf) implementation written
in C.
upb is the core runtime for protobuf languages extensions in
[Ruby](https://github.com/protocolbuffers/protobuf/tree/master/ruby),
[PHP](https://github.com/protocolbuffers/protobuf/tree/master/php), and (soon)
Python.
While upb offers a C API, the C API & ABI **are not stable**. For this reason,
upb is not generally offered as a C library for direct consumption, and there
are no releases.
## Features
upb has comparable speed to protobuf C++, but is an order of magnitude smaller
in code size.
Like the main protobuf implementation in C++, it supports:
- a generated API (in C)
- reflection
- binary & JSON wire formats
- text format serialization
- all standard features of protobufs (oneofs, maps, unknown fields, extensions,
etc.)
- full conformance with the protobuf conformance tests
upb also supports some features that C++ does not:
- **optional reflection:** generated messages are agnostic to whether
reflection will be linked in or not.
- **no global state:** no pre-main registration or other global state.
- **fast reflection-based parsing:** messages loaded at runtime parse
just as fast as compiled-in messages.
However there are a few features it does not support:
- text format parsing
- deep descriptor verification: upb's descriptor validation is not as exhaustive
as `protoc`.
## Install
For Ruby, use [RubyGems](https://rubygems.org/gems/google-protobuf):
```
$ gem install google-protobuf
```
For PHP, use [PECL](https://pecl.php.net/package/protobuf):
```
$ sudo pecl install protobuf
```
## Contributing
Please see [CONTRIBUTING.md](CONTRIBUTING.md).
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