1 ============================================= 2 Nanopb: Protocol Buffers with small code size 3 ============================================= 4 5 .. include :: menu.rst 6 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. 9 10 __ http://code.google.com/apis/protocolbuffers/ 11 12 Overall structure 13 ================= 14 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*. 17 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. 19 20 .. image:: generator_flow.png 21 22 So a typical project might include these files: 23 24 1) 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) 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) 32 33 Features and limitations 34 ======================== 35 36 **Features** 37 38 #) Pure C runtime 39 #) Small code size (210 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 59 Getting started 60 =============== 61 62 For starters, consider this simple message:: 63 64 message Example { 65 required int32 value = 1; 66 } 67 68 Save 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 73 You 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 81 Now 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 88 After that, buffer will contain the encoded message. 89 The number of bytes in the message is stored in *stream.bytes_written*. 90 You can feed the message to *protoc --decode=Example message.proto* to verify its validity. 91 92 For a complete example of the simple case, see *example/simple.c*. 93 For a more complex example with network interface, see the *example/network_server* subdirectory. 94 95 Compiler requirements 96 ===================== 97 Nanopb should compile with most ansi-C compatible compilers. It however 98 requires 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* 104 105 If these header files do not come with your compiler, you can use the 106 file *extra/pb_syshdr.h* instead. It contains an example of how to provide 107 the dependencies. You may have to edit it a bit to suit your custom platform. 108 109 To use the pb_syshdr.h, define *PB_SYSTEM_HEADER* as *"pb_syshdr.h"* (including the quotes). 110 Similarly, you can provide a custom include file, which should provide all the dependencies 111 listed above. 112 113 Running the test cases 114 ====================== 115 Extensive unittests and test cases are included under the *tests* folder. 116 117 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. 119 120 __ http://www.scons.org/ 121 122 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*. 126 127
