C++ loading data from part of a file - c++

Question

I want to keep a bunch of simple structures (just 3 ints per structure at the moment) in a file, and be able to read back just one of those structures at any given time.
As a first step, I'm trying to output them to a file, then read them back using boost::serialization. Currently I'm doing this, which crashes:
std::array<Patch, 3> outPatches;
outPatches[0].ZOrigin = 0;
outPatches[0].XOrigin = 0;
outPatches[0].Resolution = 64;
outPatches[1].ZOrigin = 1;
outPatches[1].XOrigin = 5;
outPatches[1].Resolution = 3;
outPatches[2].ZOrigin = 123;
outPatches[2].XOrigin = 546;
outPatches[2].Resolution = 6;
std::ofstream ofs("testing.sss", std::ios::binary);
for (auto const& patch : outPatches)
{
std::cout << "start archive: " << ofs.tellp() << std::endl;
{
boost::archive::binary_oarchive oa(ofs);
std::cout << "start patch: " << ofs.tellp() << std::endl;
oa << patch;
}
}
ofs.close();
std::array<Patch, 3> inPatches;
std::ifstream ifs("testing.sss", std::ios::binary);
for (auto& patch : inPatches)
{
std::cout << "start archive: " << ifs.tellg() << std::endl;
{
boost::archive::binary_iarchive ia(ifs); // <-- crash here on second patch
std::cout << "start patch: " << ifs.tellg() << std::endl;
ia >> patch;
}
}
ifs.close();
for (int i = 0; i != 3; ++i)
std::cout << "check: " << (inPatches[i] == outPatches[i]) << std::endl;
I was planning on using tell to make an index of where each structure is, and seek to skip to that structure on load. Is this a reasonable approach to take? I don't know much about streams beyond the basics.
I've tried putting all the patches in one o/iarchive instead, which works fine for reading everything sequentially. However, seeking on the stream didn't work.
I've found this, which might be what I want, but I have no idea what it's doing or how to use it, or whether it would work with boost::serialization: read part of a file with iostreams
I'd probably be willing to switch to another serialization method if necessary, since I've not got very far with this.
Edit 3: Moved edits 1 and 2 to an answer.

Answer 1

I once had a similar case (with boost / serialization). What I did back then (and it was quite efficient, if I remember) was to map the file into a virtual address, write a streamer that operates on memory buffers instead of files and for each part that I wanted to read assign appropriate offset to the streamer as buffer start / length and initialize the iarchive with the streamer so the serialization library treated it as if each object was in a separate file.
Of course, adding to the file required a re-map. Now that I look back at this, it seems a bit weird, but it was efficient, afair.

Answer 2

Boost serialization
It doesn't seem possible to skip around inside a boost serialization archive. The best I've got so far is to use multiple archives on one stream:
static const int numPatches = 5000;
std::vector<int> indices(numPatches, 0);
std::iota(indices.begin(), indices.end(), 0);
std::vector<Patch> outPatches(numPatches, Patch());
std::for_each(outPatches.begin(), outPatches.end(),
[] (Patch& p)
{
p.ZOrigin = rand();
p.XOrigin = rand();
p.Resolution = rand();
});
std::vector<int64_t> offsets(numPatches, 0);
std::ofstream ofs("testing.sss", std::ios::binary);
for (auto i : indices)
{
offsets[i] = ofs.tellp();
boost::archive::binary_oarchive oa(ofs,
boost::archive::no_header | boost::archive::no_tracking);
oa << outPatches[i];
}
ofs.close();
std::random_shuffle(indices.begin(), indices.end());
std::vector<Patch> inPatches(numPatches, Patch());
std::ifstream ifs("testing.sss", std::ios::binary);
for (auto i : indices)
{
ifs.seekg(offsets[i]);
boost::archive::binary_iarchive ia(ifs,
boost::archive::no_header | boost::archive::no_tracking);
ia >> inPatches[i];
ifs.clear();
}
std::cout << std::all_of(indices.begin(), indices.end(),
[&] (int i) { return inPatches[i] == outPatches[i]; }) << std::endl;
Unfortunately, this is very slow, so I don't think I can use it. Next up is testing protobuf.
google::protobuf
I've got something working with protobuf. It required a bit of fiddling around (apparently I have to use the LimitingInputStream type, and store the size of each object), but it's a lot faster than the boost::serialization version:
static const int numPatches = 500;
std::vector<int> indices(numPatches, 0);
std::iota(indices.begin(), indices.end(), 0);
std::vector<Patch> outPatches(numPatches, Patch());
std::for_each(outPatches.begin(), outPatches.end(),
[] (Patch& p)
{
p.ZOrigin = rand();
p.XOrigin = rand();
p.Resolution = 64;
});
std::vector<int64_t> streamOffset(numPatches, 0);
std::vector<int64_t> streamSize(numPatches, 0);
std::ofstream ofs("testing.sss", std::ios::binary);
PatchBuffer buffer;
for (auto i : indices)
{
buffer.Clear();
WriteToPatchBuffer(buffer, outPatches[i]);
streamOffset[i] = ofs.tellp();
streamSize[i] = buffer.ByteSize();
buffer.SerializeToOstream(&ofs);
}
ofs.close();
std::random_shuffle(indices.begin(), indices.end());
std::vector<Patch> inPatches(numPatches, Patch());
std::ifstream ifs("testing.sss", std::ios::binary);
for (auto i : indices)
{
ifs.seekg(streamOffset[i]);
buffer.Clear();
google::protobuf::io::IstreamInputStream iis(&ifs);
google::protobuf::io::LimitingInputStream lis(&iis, streamSize[i]);
buffer.ParseFromZeroCopyStream(&lis);
ReadFromPatchBuffer(inPatches[i], buffer);
ifs.clear();
}
std::cout << std::all_of(indices.begin(), indices.end(),
[&] (int i) { return inPatches[i] == outPatches[i]; }) << std::endl;