使用 ifstream 将二进制数据读入结构-C/C++问题

Reading binary data into struct with ifstream(使用 ifstream 将二进制数据读入结构)

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问题描述

我正在尝试使用 ifstream 从文件中读取二进制数据.

I'm trying to read binary data from a file using ifstream.

具体来说，我正在尝试用从文件中读取的数据填充这个Header"结构:

Specifically, I'm trying to populate this "Header" struct with data read from a file:

struct Header {
    char id[16];
    int length;
    int count;
};

现在，如果我以这种方式读取文件，结果正是我想要的:

Now, if I read the file in this way, the result is exactly what I want:

input.read((char*)&hdr, sizeof(hdr));

但如果我改为手动读取结构的每个变量，结果就会乱码:

But if I instead read each variable of the struct manually, the results are gibberish:

input.read((char*)&hdr.id,     sizeof(hdr.id));
input.read((char*)&hdr.length, sizeof(hdr.length));
input.read((char*)&hdr.count,  sizeof(hdr.count));

我的问题是，这里发生了什么导致这两种方法返回不同的结果?

My question is, what is happening here that makes these two methods return different results?

推荐答案

正如上面的评论所说，您可能缺少 hdr.length 和 hdr.count.我用 gcc 4.8 和 clang 3.5 试了一下，它工作正常.

As the comment above states, you are probably missing hdr.length and hdr.count. I tried it with gcc 4.8 and clang 3.5 and it works correctly.

#include <iostream>
#include <fstream>

#pragma pack(push, r1, 1)
struct Header {
    char id[15];
    int length;
    int count;
};
#pragma pack(pop, r1)

int main() {
  Header h = {"alalalala", 5, 10};

  std::fstream fh;
  fh.open("test.txt", std::fstream::out | std::fstream::binary);
  fh.write((char*)&h, sizeof(Header));
  fh.close();

  fh.open("test.txt", std::fstream::in | std::fstream::binary);

  fh.read((char*)&h.id, sizeof(h.id));
  fh.read((char*)&h.length, sizeof(h.length));
  fh.read((char*)&h.count, sizeof(h.count));

  fh.close();

  std::cout << h.id << " " << h.length << " " << h.count << std::endl;
}

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