std::istream 意外结束

Question

我正在尝试分块读取二进制文件。该文件大约为 11 kB。以下代码仅从文件中读取一次，gcount() 函数表示仅读取了 1015 个字符（字节）并且流“已耗尽”并且 while 循环不会继续（尽管缓冲区已正确填充到 @987654323 @):

std::array<int64_t, 256>  HuffmanTree::get_frequencies(std::istream & stream) const
{
    const size_t buffer_size = 4096;
    unsigned char buffer[buffer_size];
    std::array<int64_t, 256> frequencies;
    frequencies.fill(0);

    while (stream)
    {
        stream.read((char *)buffer, buffer_size);
        std::streamsize bytes_read = stream.gcount();

        for (std::streamsize i = 0; i < bytes_read; i++)
        {
            frequencies[buffer[i]]++;
        }
    }

    return frequencies;
}

是什么导致了这种行为，我该如何解决？

编辑：stream.read(...) 仅调用一次，gcount() 返回 1015。但缓冲区包含文件的前 4096 个字节（我也在 Windows 上运行 - VS 2017）

Answer 1

我相信当您要读取的字节数少于 4096 并且您调用 stream.read() 时，您的问题就会出现

我使用以下函数，尝试读取 4096，但在失败时进行整理 - 也许它有用？

// Tries to read num bytes from inRaw into buffer (space previously allocated).  When num bytes does not take
// us off the end of the file, then this does the simple obvious thing and returns true.  When num bytes takes
// us over the edge, but things are still sane (they wanted 100 bytes, only 60 bytes to go), then this fills
// as much as possible and leaves things in a nice clean state.  When there are zero bytes to go, this
// return false.
bool safeRead(
    std::ifstream& inRaw,
    char* buffer,
    uint16_t num
)
{
    auto before = inRaw.tellg();
    if (inRaw.read(buffer, num) && inRaw.good() && !inRaw.eof())
    {
        return true;
    }
    else
    {
        if (inRaw.gcount())
        {
            inRaw.clear();
            auto pos = before + std::streamoff(inRaw.gcount());
            inRaw.seekg(pos);
            return true;
        }
        else
        {
            return false;
        }
    }
}