1=============================================
2Nanopb: Protocol Buffers with small code size
3=============================================
4
5.. include :: menu.rst
6
7Nanopb is an ANSI-C library for encoding and decoding messages in Google's `Protocol Buffers`__ format with minimal requirements for RAM and code space.
8It is primarily suitable for 32-bit microcontrollers.
9
10__ http://code.google.com/apis/protocolbuffers/
11
12Overall structure
13=================
14
15For the runtime program, you always need *pb.h* for type declarations.
16Depending on whether you want to encode, decode, or both, you also need *pb_encode.h/c* or *pb_decode.h/c*.
17
18The 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.
19
20.. image:: generator_flow.png
21
22So a typical project might include these files:
23
241) Nanopb runtime library:
25    - pb.h
26    - pb_decode.h and pb_decode.c (needed for decoding messages)
27    - pb_encode.h and pb_encode.c (needed for encoding messages)
282) Protocol description (you can have many):
29    - person.proto (just an example)
30    - person.pb.c (autogenerated, contains initializers for const arrays)
31    - person.pb.h (autogenerated, contains type declarations)
32
33Features and limitations
34========================
35
36**Features**
37
38#) Pure C runtime
39#) Small code size (2–10 kB depending on processor, plus any message definitions)
40#) Small ram usage (typically ~300 bytes, plus any message structs)
41#) Allows specifying maximum size for strings and arrays, so that they can be allocated statically.
42#) No malloc needed: everything can be allocated statically or on the stack.
43#) You can use either encoder or decoder alone to cut the code size in half.
44#) Support for most protobuf features, including: all data types, nested submessages, default values, repeated and optional fields, packed arrays, extension fields.
45#) Callback mechanism for handling messages larger than can fit in available RAM.
46#) Extensive set of tests.
47
48**Limitations**
49
50#) Some speed has been sacrificed for code size.
51#) Encoding is focused on writing to streams. For memory buffers only it could be made more efficient.
52#) The deprecated Protocol Buffers feature called "groups" is not supported.
53#) Fields in the generated structs are ordered by the tag number, instead of the natural ordering in .proto file.
54#) Unknown fields are not preserved when decoding and re-encoding a message.
55#) Reflection (runtime introspection) is not supported. E.g. you can't request a field by giving its name in a string.
56#) 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.
57#) Cyclic references between messages are supported only in callback mode.
58
59Getting started
60===============
61
62For starters, consider this simple message::
63
64 message Example {
65    required int32 value = 1;
66 }
67
68Save this in *message.proto* and compile it::
69
70    user@host:~$ protoc -omessage.pb message.proto
71    user@host:~$ python nanopb/generator/nanopb_generator.py message.pb
72
73You should now have in *message.pb.h*::
74
75 typedef struct {
76    int32_t value;
77 } Example;
78
79 extern const pb_field_t Example_fields[2];
80
81Now in your main program do this to encode a message::
82
83 Example mymessage = {42};
84 uint8_t buffer[10];
85 pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
86 pb_encode(&stream, Example_fields, &mymessage);
87
88After that, buffer will contain the encoded message.
89The number of bytes in the message is stored in *stream.bytes_written*.
90You can feed the message to *protoc --decode=Example message.proto* to verify its validity.
91
92For a complete example of the simple case, see *example/simple.c*.
93For a more complex example with network interface, see the *example/network_server* subdirectory.
94
95Compiler requirements
96=====================
97Nanopb should compile with most ansi-C compatible compilers. It however
98requires a few header files to be available:
99
100#) *string.h*, with these functions: *strlen*, *memcpy*, *memset*
101#) *stdint.h*, for definitions of *int32_t* etc.
102#) *stddef.h*, for definition of *size_t*
103#) *stdbool.h*, for definition of *bool*
104
105If these header files do not come with your compiler, you can use the
106file *extra/pb_syshdr.h* instead. It contains an example of how to provide
107the dependencies. You may have to edit it a bit to suit your custom platform.
108
109To use the pb_syshdr.h, define *PB_SYSTEM_HEADER* as *"pb_syshdr.h"* (including the quotes).
110Similarly, you can provide a custom include file, which should provide all the dependencies
111listed above.
112
113Running the test cases
114======================
115Extensive unittests and test cases are included under the *tests* folder.
116
117To build the tests, you will need the `scons`__ build system. The tests should
118be runnable on most platforms. Windows and Linux builds are regularly tested.
119
120__ http://www.scons.org/
121
122In addition to the build system, you will also need a working Google Protocol
123Buffers *protoc* compiler, and the Python bindings for Protocol Buffers. On
124Debian-based systems, install the following packages: *protobuf-compiler*,
125*python-protobuf* and *libprotobuf-dev*.
126
127