Method ReadFromFile

util/system/direct_io.cpp:171–219

}

size_t TDirectIOBufferedFile::ReadFromFile(void* buffer, size_t byteCount, ui64 offset) {
    SetDirectIO(true);

    ui64 bytesRead = 0;

    while (byteCount) {
        if (!Alignment || IsAligned(buffer) && IsAligned(byteCount) && IsAligned(offset)) {
            if (const ui64 fromFile = PreadSafe(buffer, byteCount, offset)) {
                buffer = (char*)buffer + fromFile;
                byteCount -= fromFile;
                offset += fromFile;
                bytesRead += fromFile;
            } else {
                return bytesRead;
            }
        } else {
            break;
        }
    }

    if (!byteCount) {
        return bytesRead;
    }

    ui64 bufSize = AlignUp(Min<size_t>(BufferStorage.Size(), byteCount + (Alignment << 1)), Alignment);
    TBuffer readBufferStorage(bufSize + Alignment);
    char* readBuffer = AlignUp((char*)readBufferStorage.Data(), Alignment);

    while (byteCount) {
        ui64 begin = AlignDown(offset, (ui64)Alignment);
        ui64 end = AlignUp(offset + byteCount, (ui64)Alignment);
        ui64 toRead = Min(end - begin, bufSize);
        ui64 fromFile = PreadSafe(readBuffer, toRead, begin);

        if (!fromFile) {
            break;
        }

        ui64 delta = offset - begin;
        ui64 count = Min<ui64>(fromFile - delta, byteCount);

        memcpy(buffer, readBuffer + delta, count);
        buffer = (char*)buffer + count;
        byteCount -= count;
        offset += count;
        bytesRead += count;
    }
    return bytesRead;
}

size_t TDirectIOBufferedFile::Read(void* buffer, size_t byteCount) {
    size_t bytesRead = Pread(buffer, byteCount, ReadPosition);