third_party/nanopb/docs/index.rst

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Nanopb: Protocol Buffers with small code size
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.. include :: menu.rst

Nanopb is an ANSI-C library for encoding and decoding messages in Google's `Protocol Buffers`__ format with minimal requirements for RAM and code space.
It is primarily suitable for 32-bit microcontrollers.

__ http://code.google.com/apis/protocolbuffers/

Overall structure
=================

For the runtime program, you always need *pb.h* for type declarations.
Depending on whether you want to encode, decode, or both, you also need *pb_encode.h/c* or *pb_decode.h/c*.

The high-level encoding and decoding functions take an array of *pb_field_t* structures, which describes the fields of a message structure. Usually you want these autogenerated from a *.proto* file. The tool script *nanopb_generator.py* accomplishes this.

.. image:: generator_flow.png

So a typical project might include these files:

1) Nanopb runtime library:
    - pb.h
    - pb_decode.h and pb_decode.c (needed for decoding messages)
    - pb_encode.h and pb_encode.c (needed for encoding messages)
2) Protocol description (you can have many):
    - person.proto (just an example)
    - person.pb.c (autogenerated, contains initializers for const arrays)
    - person.pb.h (autogenerated, contains type declarations)

Features and limitations
========================

**Features**

#) Pure C runtime
#) Small code size (2–10 kB depending on processor, plus any message definitions)
#) Small ram usage (typically ~300 bytes, plus any message structs)
#) Allows specifying maximum size for strings and arrays, so that they can be allocated statically.
#) No malloc needed: everything can be allocated statically or on the stack.
#) You can use either encoder or decoder alone to cut the code size in half.
#) Support for most protobuf features, including: all data types, nested submessages, default values, repeated and optional fields, packed arrays, extension fields.
#) Callback mechanism for handling messages larger than can fit in available RAM.
#) Extensive set of tests.

**Limitations**

#) Some speed has been sacrificed for code size.
#) Encoding is focused on writing to streams. For memory buffers only it could be made more efficient.
#) The deprecated Protocol Buffers feature called "groups" is not supported.
#) Fields in the generated structs are ordered by the tag number, instead of the natural ordering in .proto file.
#) Unknown fields are not preserved when decoding and re-encoding a message.
#) Reflection (runtime introspection) is not supported. E.g. you can't request a field by giving its name in a string.
#) Numeric arrays are always encoded as packed, even if not marked as packed in .proto. This causes incompatibility with decoders that do not support packed format.
#) Cyclic references between messages are supported only in callback mode.

Getting started
===============

For starters, consider this simple message::

 message Example {
    required int32 value = 1;
 }

Save this in *message.proto* and compile it::

    user@host:~$ protoc -omessage.pb message.proto
    user@host:~$ python nanopb/generator/nanopb_generator.py message.pb

You should now have in *message.pb.h*::

 typedef struct {
    int32_t value;
 } Example;
 
 extern const pb_field_t Example_fields[2];

Now in your main program do this to encode a message::

 Example mymessage = {42};
 uint8_t buffer[10];
 pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
 pb_encode(&stream, Example_fields, &mymessage);

After that, buffer will contain the encoded message.
The number of bytes in the message is stored in *stream.bytes_written*.
You can feed the message to *protoc --decode=Example message.proto* to verify its validity.

For a complete example of the simple case, see *example/simple.c*.
For a more complex example with network interface, see the *example/network_server* subdirectory.

Compiler requirements
=====================
Nanopb should compile with most ansi-C compatible compilers. It however
requires a few header files to be available:

#) *string.h*, with these functions: *strlen*, *memcpy*, *memset*
#) *stdint.h*, for definitions of *int32_t* etc.
#) *stddef.h*, for definition of *size_t*
#) *stdbool.h*, for definition of *bool*

If these header files do not come with your compiler, you can use the
file *extra/pb_syshdr.h* instead. It contains an example of how to provide
the dependencies. You may have to edit it a bit to suit your custom platform.

To use the pb_syshdr.h, define *PB_SYSTEM_HEADER* as *"pb_syshdr.h"* (including the quotes).
Similarly, you can provide a custom include file, which should provide all the dependencies
listed above.

Running the test cases
======================
Extensive unittests and test cases are included under the *tests* folder.

To build the tests, you will need the `scons`__ build system. The tests should
be runnable on most platforms. Windows and Linux builds are regularly tested.

__ http://www.scons.org/

In addition to the build system, you will also need a working Google Protocol
Buffers *protoc* compiler, and the Python bindings for Protocol Buffers. On
Debian-based systems, install the following packages: *protobuf-compiler*,
*python-protobuf* and *libprotobuf-dev*.