1 =============================================
2 Nanopb: Protocol Buffers with small code size
3 =============================================
7 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.
8 It is primarily suitable for 32-bit microcontrollers.
10 __ http://code.google.com/apis/protocolbuffers/
15 For the runtime program, you always need *pb.h* for type declarations.
16 Depending on whether you want to encode, decode, or both, you also need *pb_encode.h/c* or *pb_decode.h/c*.
18 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.
20 .. image:: generator_flow.png
22 So a typical project might include these files:
24 1) Nanopb runtime library:
26 - pb_decode.h and pb_decode.c (needed for decoding messages)
27 - pb_encode.h and pb_encode.c (needed for encoding messages)
28 2) 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)
33 Features and limitations
34 ========================
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.
50 #) User must provide callbacks when decoding arrays or strings without maximum size. Malloc support could be added as a separate module.
51 #) Some speed has been sacrificed for code size.
52 #) Encoding is focused on writing to streams. For memory buffers only it could be made more efficient.
53 #) The deprecated Protocol Buffers feature called "groups" is not supported.
54 #) Fields in the generated structs are ordered by the tag number, instead of the natural ordering in .proto file.
55 #) Unknown fields are not preserved when decoding and re-encoding a message.
56 #) Reflection (runtime introspection) is not supported. E.g. you can't request a field by giving its name in a string.
57 #) 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.
58 #) Cyclic references between messages are supported only in callback mode.
63 For starters, consider this simple message::
66 required int32 value = 1;
69 Save this in *message.proto* and compile it::
71 user@host:~$ protoc -omessage.pb message.proto
72 user@host:~$ python nanopb/generator/nanopb_generator.py message.pb
74 You should now have in *message.pb.h*::
80 extern const pb_field_t Example_fields[2];
82 Now in your main program do this to encode a message::
84 Example mymessage = {42};
86 pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
87 pb_encode(&stream, Example_fields, &mymessage);
89 After that, buffer will contain the encoded message.
90 The number of bytes in the message is stored in *stream.bytes_written*.
91 You can feed the message to *protoc --decode=Example message.proto* to verify its validity.
93 For a complete example of the simple case, see *example/simple.c*.
94 For a more complex example with network interface, see the *example/network_server* subdirectory.
98 Nanopb should compile with most ansi-C compatible compilers. It however
99 requires a few header files to be available:
101 #) *string.h*, with these functions: *strlen*, *memcpy*, *memset*
102 #) *stdint.h*, for definitions of *int32_t* etc.
103 #) *stddef.h*, for definition of *size_t*
104 #) *stdbool.h*, for definition of *bool*
106 If these header files do not come with your compiler, you can use the
107 file *compat/pb_syshdr.h* instead. It contains an example of how to provide
108 the dependencies. You may have to edit it a bit to suit your custom platform.
110 To use the pb_syshdr.h, define *PB_SYSTEM_HEADER* to be the name of your custom
111 header file. It should provide all the dependencies listed above.
113 Running the test cases
114 ======================
115 Extensive unittests and test cases are included under the *tests* folder.
117 To build the tests, you will need the `scons`__ build system. The tests should
118 be runnable on most platforms. Windows and Linux builds are regularly tested.
120 __ http://www.scons.org/
122 In addition to the build system, you will also need a working Google Protocol
123 Buffers *protoc* compiler, and the Python bindings for Protocol Buffers. On
124 Debian-based systems, install the following packages: *protobuf-compiler*,
125 *python-protobuf* and *libprotobuf-dev*.