C++ 从二进制文件中读回 "incorrect"值？

Question

我正在处理的项目，作为一种自定义文件格式，由几个不同变量的标题组成，后跟像素数据。我的同事开发了一个 GUI，可以很好地处理、写入、读取和显示这种类型的文件格式。

但我的问题是，虽然我协助编写了将数据写入磁盘的代码，但我无法自己读取这种文件并获得令人满意的值。我能够读回第一个变量(字符数组)，但不能读出以下值。

所以文件格式符合以下结构:

typedef struct {
    char hxtLabel[8];
    u64 hxtVersion;
    int motorPositions[9];
    int filePrefixLength;
    char filePrefix[100];
..
} HxtBuffer;

在代码中，我创建了一个具有上述结构的对象，然后设置了这些示例值:

setLabel("MY_LABEL");
setFormatVersion(3);
setMotorPosition( 2109, 5438, 8767, 1234, 1022, 1033, 1044, 1055, 1066);
setFilePrefixLength(7);
setFilePrefix( string("prefix_"));
setDataTimeStamp( string("000000_000000"));

我打开文件的代码:

// Open data file, binary mode, reading
ifstream datFile(aFileName.c_str(), ios::in | ios::binary);
if (!datFile.is_open()) {
    cout  << "readFile() ERROR: Failed to open file " << aFileName << endl;
    return false;
}

// How large is the file?
datFile.seekg(0, datFile.end);
int length =  datFile.tellg();
datFile.seekg(0, datFile.beg);

cout << "readFile() file " << setw(70) << aFileName << " is: " << setw(15) << length  << " long\n";

// Allocate memory for buffer:
char * buffer = new char[length];
// Read data as one block:
datFile.read(buffer, length);
datFile.close();

/// Looking at the start of the buffer, I should be seeing  "MY_LABEL"?

cout << "buffer: " << buffer  << " " << *(buffer)  << endl;

int* mSSX = reinterpret_cast<int*>(*(buffer+8));
int* mSSY = reinterpret_cast<int*>(&buffer+9);
int* mSSZ = reinterpret_cast<int*>(&buffer+10);
int* mSSROT = reinterpret_cast<int*>(&buffer+11);
int* mTimer = reinterpret_cast<int*>(&buffer+12);
int* mGALX = reinterpret_cast<int*>(&buffer+13);
int* mGALY = reinterpret_cast<int*>(&buffer+14);
int* mGALZ = reinterpret_cast<int*>(&buffer+15);
int* mGALROT = reinterpret_cast<int*>(&buffer+16);
int* filePrefixLength = reinterpret_cast<int*>(&buffer+17);

std::string filePrefix;   std::string dataTimeStamp;

// Read file prefix character by character into stringstream object
std::stringstream ss;
char* cPointer = (char *)(buffer+18);
int k;
for(k = 0; k < *filePrefixLength; k++)
{
    //read string
    char c;
    c = *cPointer;
    ss << c;
    cPointer++;
}
filePrefix = ss.str();

// Read timestamp character by character into stringstream object
std::stringstream timeStampStream;
/// Need not increment cPointer, already pointing @ 1st char of timeStamp
for (int l= 0; l < 13; l++)
{
    char c;
    c = * cPointer;
    timeStampStream << c;
}
dataTimeStamp = timeStampStream.str();

cout << 25 << endl;
cout << " mSSX:   "  << mSSX <<   "  mSSY:   "  << mSSY <<      "  mSSZ: "  << mSSZ;
cout << " mSSROT: "  << mSSROT << "  mTimer: "  << mTimer <<    "  mGALX: "  << mGALX;
cout << " mGALY:  "  << mGALY <<  "  mGALZ:  "  << mGALZ <<     "  mGALROT: "  << mGALROT;

最后，我在下面看到了。我添加 25 只是为了仔细检查并非所有内容都是十六进制的。如您所见，我能够按预期看到标签“MY_LABEL”。但是 9 个 motorPositions 都看起来可疑地看起来地址不是值。文件前缀和数据时间戳(应该是字符串，或者至少是字符)都是空的。

buffer: MY_LABEL M
25
 mSSX:   0000000000000003  mSSY:   00000000001BF618  mSSZ: 00000000001BF620 mSSROT: 00000000001BF628  mTimer: 00000000001BF630  mGALX: 00000000001BF638 mGALY:  00000000001BF640  mGALZ:  00000000001BF648  mGALROT: 00000000001BF650filePrefix: dataTimeStamp: 

我确信解决方案不会太复杂，但我到了一个我只是在旋转，无法理解事物的阶段。

非常感谢您阅读这篇有点长的文章。

-- 编辑--

我可能会达到帖子允许的最大长度，但以防万一我认为我应该发布生成我试图回读的数据的代码:

bool writePixelOutput(string aOutputPixelFileName) {

    // Write pixel histograms out to binary file
    ofstream pixelFile;
    pixelFile.open(aOutputPixelFileName.c_str(), ios::binary | ios::out | ios::trunc);
    if (!pixelFile.is_open()) {
        LOG(gLogConfig, logERROR) << "Failed to open output file " << aOutputPixelFileName;
        return false;
    }

    // Write binary file header

    string label("MY_LABEL");
    pixelFile.write(label.c_str(), label.length());

    pixelFile.write((const char*)&mFormatVersion, sizeof(u64));

    // Include File Prefix/Motor Positions/Data Time Stamp - if format version > 1
    if (mFormatVersion > 1)
    {
        pixelFile.write((const char*)&mSSX, sizeof(mSSX));
        pixelFile.write((const char*)&mSSY, sizeof(mSSY));
        pixelFile.write((const char*)&mSSZ, sizeof(mSSZ));
        pixelFile.write((const char*)&mSSROT, sizeof(mSSROT));
        pixelFile.write((const char*)&mTimer, sizeof(mTimer));
        pixelFile.write((const char*)&mGALX, sizeof(mGALX));
        pixelFile.write((const char*)&mGALY, sizeof(mGALY));
        pixelFile.write((const char*)&mGALZ, sizeof(mGALZ));
        pixelFile.write((const char*)&mGALROT, sizeof(mGALROT));

        // Determine length of mFilePrefix string
        int filePrefixSize = (int)mFilePrefix.size();

        // Write prefix length, followed by prefix itself
        pixelFile.write((const char*)&filePrefixSize, sizeof(filePrefixSize));

        size_t prefixLen = 0;
        if (mFormatVersion == 2)    prefixLen = mFilePrefix.size();
        else                        prefixLen = 100;
        pixelFile.write(mFilePrefix.c_str(), prefixLen);

        pixelFile.write(mDataTimeStamp.c_str(), mDataTimeStamp.size());
    }
    // Continue writing header information that is common to both format versions
    pixelFile.write((const char*)&mRows, sizeof(mRows));
    pixelFile.write((const char*)&mCols, sizeof(mCols));
    pixelFile.write((const char*)&mHistoBins, sizeof(mHistoBins));

    // Write the actual data - taken out for briefy sake
    // ..

    pixelFile.close();

    LOG(gLogConfig, logINFO) << "Written output histogram binary file " << aOutputPixelFileName;

    return true;
}

-- 编辑 2 (11:32 09/12/2015) --

感谢您的所有帮助，我现在离解决问题更近了。根据 muelleth 的回答，我尝试:

/// Read into char buffer
char * buffer = new char[length];
datFile.read(buffer, length);// length determined by ifstream.seekg()

/// Let's try HxtBuffer
HxtBuffer *input = new HxtBuffer;
cout << "sizeof HxtBuffer:  " << sizeof *input << endl;
memcpy(input, buffer, length); 

然后我可以显示不同的结构变量:

qDebug() << "Slice BUFFER label " << QString::fromStdString(input->hxtLabel);
qDebug() << "Slice BUFFER version " << QString::number(input->hxtVersion);
qDebug() << "Slice BUFFER hxtPrefixLength " << QString::number(input->filePrefixLength);
for (int i = 0; i < 9; i++)
{
    qDebug() << i << QString::number(input->motorPositions[i]);
}
qDebug() << "Slice BUFFER filePrefix " << QString::fromStdString(input->filePrefix);
qDebug() << "Slice BUFFER dataTimeStamp " << QString::fromStdString(input->dataTimeStamp);
qDebug() << "Slice BUFFER nRows " << QString::number(input->nRows);
qDebug() << "Slice BUFFER nCols " << QString::number(input->nCols);
qDebug() << "Slice BUFFER nBins " << QString::number(input->nBins);

然后输出基本符合预期:

Slice BUFFER label  "MY_LABEL" 
Slice BUFFER version  "3" 
Slice BUFFER hxtPrefixLength  "2" 
0 "2109" 
1 "5438" 
...
7 "1055" 
8 "1066" 
Slice BUFFER filePrefix  "-1" 
Slice BUFFER dataTimeStamp  "000000_000000P" 
Slice BUFFER nRows  "20480" 
Slice BUFFER nCols  "256000" 
Slice BUFFER nBins  "0" 

EXCEPT，长度为 13 个字符的 dataTimeStamp 显示为 14 个字符。后面的 3 个变量:nRows、nCols 和 nBins 是不正确的。 (应该是 nRows=80，nCols=80，nBins=1000)。我的猜测是属于 dataTimeStamp 第 14 个字符的位应该与 nRows 一起读取，因此级联以产生正确的 nCols和 nBins。

我已经使用 qDebug 单独验证(此处未显示)我写入文件的内容确实是我期望的值及其各自的大小。

Answer 1

我个人会尝试从文件中准确读取您的结构的字节数，即类似于

int length = sizeof(HxtBuffer);

然后简单地使用 memcpy 从读取缓冲区分配一个本地结构:

HxtBuffer input;
memcpy(&input, buffer, length);

然后您可以访问您的数据，例如喜欢:

std::cout << "Data: " << input.hxtLabel << std::endl;