#include <sstream>
#include <istream>
#include <iostream>

void fill_testdata(std::ostream& os);

struct FormatData {
    std::string signature, header; // e.g. 4 + 16 = 20 bytes - could be different, of course
    std::string payload;           // 16bit length prefixed
};

FormatData parse(std::istream& is);

int main() {
    std::stringstream ss;
    fill_testdata(ss);

    try {
        FormatData data = parse(ss);

        std::cout << "actual payload bytes: " << data.payload.length() << "\n";
        std::cout << "payload: '" << data.payload << "'\n";
    } catch(std::runtime_error const& e) {
        std::cout << "Error: " << e.what() << "\n";
    }
}

// some testdata
void fill_testdata(std::ostream& os)
{
    char data[] = { 
        'S'   , 'I'   , 'G'   , 'N'   , '\x00'   , // 0..4
        '\x00', '\x00', '\x00', '\x00', '\x00'   , // 5..9
        '\x00', '\x00', '\x00', '\x00', '\x00'   , // 10..14
        '\x00', '\x00', '\x00', '\x00', '\x00'   , // 15..19
        '\x0b', '\x00', 'H'   , 'e'   , 'l'      , // 20..24
        'l'   , 'o'   , ' '   , 'w'   , 'o'      , // 25..29
        'r'   , 'l'   , 'd'   , '!'   , // 30..33
    };
    os.write(data, sizeof(data));
}

//#define BOOST_SPIRIT_DEBUG
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;

BOOST_FUSION_ADAPT_STRUCT(FormatData, signature, header, payload)

template <typename It>
struct FileFormat : qi::grammar<It, FormatData()> {
    FileFormat() : FileFormat::base_type(start) {
        using namespace qi;

        signature  = string("SIGN");     // 4 byte signature, just for example
        header     = repeat(16) [byte_]; // 16 byte header, same

        payload   %= omit[little_word[_len=_1]] >> repeat(_len) [byte_];
        start      = signature >> header >> payload;

        //BOOST_SPIRIT_DEBUG_NODES((start)(signature)(header)(payload))
    }
  private:
    qi::rule<It, FormatData()> start;
    qi::rule<It, std::string()> signature, header;

    qi::_a_type _len;
    qi::rule<It, std::string(), qi::locals<uint16_t> > payload;
};

FormatData parse(std::istream& is) {
    using it = boost::spirit::istream_iterator;

    FormatData data;
    it f(is >> std::noskipws), l;
    bool ok = parse(f, l, FileFormat<it>{}, data);

    if (!ok)
        throw std::runtime_error("parse failure\n");

    return data;
}