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diff --git a/docs/index.rst b/docs/index.rst new file mode 100644 index 0000000..d49abc0 --- /dev/null +++ b/docs/index.rst @@ -0,0 +1,126 @@ +============================================= +Nanopb: Protocol Buffers with small code size +============================================= + +.. 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*. + |