Combining stream_file::seek() and stream_file::read_some() behaves unexpectedly on Windows

I'm having a problem with stream_file when trying to read some data, then skip a section of a file. The basic pattern looks like this:

while (...) {
    asio::read(file, asio::buffer(buf));
    file.seek(N, asio::file_base::seek_cur);
}

The problem is that the file position for calls to read() and seek() seem to be separated to a degree. Two calls to seek(N, seek_cur) behave as if the file position had not changed from the previous call to seek(), regardless of the presence of any number of read operations between the two calls.

For example, this bit of code

auto context = asio::io_context{};
auto file = asio::stream_file{context, "file.bin"};
auto buf = std::array<char, 4>{};

while (true) {
    std::cout << file.seek(0, asio::file_base::seek_cur) << std::endl;
    asio::read(file, asio::buffer(buf));
    file.seek(100, asio::file_base::seek_cur);
}

would produce this output

but I would expect it to be this

If this is unintended, then I suspect the cause might be that the file pointer is not updated by a call to ReadFile for a file opened in overlapped mode, so the call to SetFilePointerEx is still working with the value set from the previous invocation. If this is actually intentional, then I believe it's quite unexpected and could stand to be explicitly mentioned in the documentation.

chriskohlhoff / asio

Combining stream_file::seek() and stream_file::read_some() behaves unexpectedly on Windows #1222