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__ https://developers.google.com/protocol-buffers/docs/reference/overview 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_common.h and pb_common.c (always needed) 27 - pb_decode.h and pb_decode.c (needed for decoding messages) 28 - pb_encode.h and pb_encode.c (needed for encoding messages) 292) Protocol description (you can have many): 30 - person.proto (just an example) 31 - person.pb.c (autogenerated, contains initializers for const arrays) 32 - person.pb.h (autogenerated, contains type declarations) 33 34Features and limitations 35======================== 36 37**Features** 38 39#) Pure C runtime 40#) Small code size (2–10 kB depending on processor, plus any message definitions) 41#) Small ram usage (typically ~300 bytes, plus any message structs) 42#) Allows specifying maximum size for strings and arrays, so that they can be allocated statically. 43#) No malloc needed: everything can be allocated statically or on the stack. Optional malloc support available. 44#) You can use either encoder or decoder alone to cut the code size in half. 45#) Support for most protobuf features, including: all data types, nested submessages, default values, repeated and optional fields, oneofs, packed arrays, extension fields. 46#) Callback mechanism for handling messages larger than can fit in available RAM. 47#) Extensive set of tests. 48 49**Limitations** 50 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. 58#) Cyclic references between messages are supported only in callback and malloc mode. 59 60Getting started 61=============== 62 63For starters, consider this simple message:: 64 65 message Example { 66 required int32 value = 1; 67 } 68 69Save this in *message.proto* and compile it:: 70 71 user@host:~$ protoc -omessage.pb message.proto 72 user@host:~$ python nanopb/generator/nanopb_generator.py message.pb 73 74You should now have in *message.pb.h*:: 75 76 typedef struct { 77 int32_t value; 78 } Example; 79 80 extern const pb_field_t Example_fields[2]; 81 82Now in your main program do this to encode a message:: 83 84 Example mymessage = {42}; 85 uint8_t buffer[10]; 86 pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer)); 87 pb_encode(&stream, Example_fields, &mymessage); 88 89After that, buffer will contain the encoded message. 90The number of bytes in the message is stored in *stream.bytes_written*. 91You can feed the message to *protoc --decode=Example message.proto* to verify its validity. 92 93For a complete example of the simple case, see *example/simple.c*. 94For a more complex example with network interface, see the *example/network_server* subdirectory. 95 96Compiler requirements 97===================== 98Nanopb should compile with most ansi-C compatible compilers. It however 99requires a few header files to be available: 100 101#) *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* 105 106If these header files do not come with your compiler, you can use the 107file *extra/pb_syshdr.h* instead. It contains an example of how to provide 108the dependencies. You may have to edit it a bit to suit your custom platform. 109 110To use the pb_syshdr.h, define *PB_SYSTEM_HEADER* as *"pb_syshdr.h"* (including the quotes). 111Similarly, you can provide a custom include file, which should provide all the dependencies 112listed above. 113 114Running the test cases 115====================== 116Extensive unittests and test cases are included under the *tests* folder. 117 118To build the tests, you will need the `scons`__ build system. The tests should 119be runnable on most platforms. Windows and Linux builds are regularly tested. 120 121__ http://www.scons.org/ 122 123In addition to the build system, you will also need a working Google Protocol 124Buffers *protoc* compiler, and the Python bindings for Protocol Buffers. On 125Debian-based systems, install the following packages: *protobuf-compiler*, 126*python-protobuf* and *libprotobuf-dev*. 127 128
