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author | Petteri Aimonen <jpa@npb.mail.kapsi.fi> | 2011-08-10 20:08:06 +0000 |
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committer | Petteri Aimonen <jpa@npb.mail.kapsi.fi> | 2011-08-10 20:08:06 +0000 |
commit | 09f92bafa59460ea4597c557e469e982386c9e3b (patch) | |
tree | a5949245e6544eb5795e9b71f4df112ec52b31d3 /docs/index.rst | |
parent | 0c5bc4caf9cfb221822fead442b462be2f47b97b (diff) |
Started writing documentation
git-svn-id: https://svn.kapsi.fi/jpa/nanopb@953 e3a754e5-d11d-0410-8d38-ebb782a927b9
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diff --git a/docs/index.rst b/docs/index.rst new file mode 100644 index 0000000..93b06e8 --- /dev/null +++ b/docs/index.rst @@ -0,0 +1,92 @@ +============================================= +Nanopb: Protocol Buffers with small code size +============================================= + +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 string *nanopb_generator.py* accomplishes this. + +So a typical project might include these files: + +1) Nanopb runtime library: + - pb.h + - pb_decode.h and pb_decode.c + - pb_encode.h and pb_encode.c +2) Protocol description (you can have many): + - person.proto + - person.c (autogenerated, contains initializers for const arrays) + - person.h (autogenerated, contains type declarations) + +Features and limitations +======================== + +**Features** + +#) Pure C runtime +#) Small code size (2–10 kB depending on processor) +#) Small ram usage (typically 200 bytes) +#) Allows specifying maximum size for strings and arrays, so that they can be allocated statically. +#) No malloc needed: everything is stored on the stack. +#) You can use either encoder or decoder alone to cut the code size in half. + +**Limitations** + +#) User must provide callbacks when decoding arrays or strings without maximum size. +#) Some speed has been sacrificed for code size. For example varint calculations are always done in 64 bits. +#) 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. + +Getting started +=============== + +For starters, consider this simple message:: + + message Example { + required int32 value = 1; + } + +Save this in *example.proto* and run it through *nanopb_generate.py*. You +should now have in *example.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 example.proto* to verify its validity. + +Library reference +================= + +**Encoding** + +**Decoding** + +**Specifying field options** + +**Generated code** + +Wishlist +======== +#) A specialized encoder for encoding to a memory buffer. Should serialize in reverse order to avoid having to determine submessage size beforehand. +#) A cleaner rewrite of the source generator. +#) Better performance for 16- and 8-bit platforms. |