如何在 c++ 中读取二进制文件中的小端整数-解网

问：

我一直在尝试读取一个小的字节序二进制文件。我希望首先在文件中找到一个字符串的索引，从那里我可以开始读取数据。一旦我得到索引，我将寻求提取学生信息。我已经成功地阅读了字符串（名字和姓氏）。但是，我无法提取整数（年龄）。感谢您的帮助:) 下面是二进制文件中学生信息部分的示例

130000004A4F484E444F45

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

class BinaryData
{
private:
   string   age;
   string   firstName;
   string   lastName;  
public:
   BinaryData(){}
   ~BinaryData(){}
   void readStudentData(string fileName)
   {
      //open the file and get its size
      ifstream in(fileName.c_str(), ios::in | ios::binary);
      in.seekg(0, ios::end);
      int fileSize = in.tellg();
      in.seekg(0, ios::beg);

      //read the entire file into a buffer
      string studentBuffer;
      studentBuffer.resize(fileSize);
      in.read(&studentBuffer[0], studentBuffer.size());
      
      //move to the position where to start reading student info
      int index = studentBuffer.find("StudentInfo");
      if(index != 0 && index != std::string::npos)
      {
          //read the student info if the header is found
          age = studentBuffer.substr(index, 4); //assume we are dedicating 4 bytes for age
          index += 4;
          firstName = studentBuffer.substr(index, 10); //assume we are dedicating 10 bytes for first name
          index += 10;
          lastName = studentBuffer.substr(index, 10); //assume we are dedicating 10 bytes for last name 
          std::cout<< "The student name is: " << firstName << " " << lastName << ", they are " << age << " years old" << std::endl;
      }
      in.close();
   }
}

我的代码的输出将是：

The student name is: John Doe, they are **(some character)** years old

预期输出应为：

The student name is: John Doe, they are 19 years old

年龄数据如下所示：二进制文件中的 13 00 00 00，应转换为 19，因为这是小端序。

任何帮助将不胜感激。谢谢！

C++ 二进制文件字节序

#include <iostream>
#include <string>
#include <cstdint>
#include <stdexcept>

/**
 * Converts a string containing binary data into a 64-bit unsigned integer.
 * The string is interpreted in little endian format.
 * If the string contains fewer than 1 byte or more than 8 bytes, an exception is thrown.
 * 
 * @param binary_data The string containing the binary data.
 * @return The 64-bit unsigned integer representation of the binary data.
 */
uint64_t convertToUInt64(const std::string& binary_data) {
    if (binary_data.empty()) {
        throw std::invalid_argument("Input string must contain at least 1 byte.");
    }
    if (binary_data.size() > sizeof(uint64_t)) {
        throw std::invalid_argument("Input string must contain no more than 8 bytes.");
    }

    uint64_t value = 0;

    // Iterate over the string to construct the integer
    for (size_t i = 0; i < binary_data.size(); ++i) {
        // Cast each character to an unsigned byte and then shift it to the correct position
        value |= static_cast<uint64_t>(static_cast<uint8_t>(binary_data[i])) << (i * 8);
    }

    return value;
}

您可以使用示例二进制数据在 a 中调用它：string

int main() {
    // Binary data in a string, representing the value 19 in 3 bytes (little endian)
    std::string binary_data = "\x13\x00\x00";

    // Convert the binary data to a 64-bit unsigned integer
    uint64_t value = convertToUInt64(binary_data);

    // Output the integer value
    std::cout << "The 64-bit unsigned integer value is: " << value << std::endl;

    return 0;
}

我展示了一个泛型函数，而不是一个只接受 3 个字节的函数，以防还使用其他大小的字符串。如果想要较小的整数，可以检查整数是否低于特定大小，然后进行赋值。

1赞 selbie 11/10/2023 #2

一种可移植的方式，用于读取 2 的补码架构的小端序（这几乎是每台现代计算机）。

将 4 个字节：（如下所示）从字符缓冲区中读取为整数。13 00 00 00

uint32_t age;
memcpy(&age, studentBuffer.c_str()+index, 4);
index += 4;

如果您使用的是英特尔，那么您可能已经完成了。由于英特尔处理器是 Little Endian。将具有预期值。或者在本例中，0x13 == 十进制 19。age

如果你想让你的代码在 big-endian 和 little-endian 硬件上运行，你可以这样做：

 if (isBigEndian()) {
      // swap bytes
      uint32_t b1 = (age >> 24) & 0x000000ff;
      uint32_t b2 = (age >> 8) &  0x0000ff00;
      uint32_t b3 = (age << 8) &  0x00ff0000;
      uint32_t b4 = (age << 24) & 0xff000000;
      age = b1|b2|b3|b4;
 }

哪里可以写成如下：isBigEndian()

bool isBigEndian() {
    uint8_t buffer[4] = {0};
    uint32_t t = 1;
    memcpy(buffer, &t, 4);
    return (buffer[0] == 0);
}

如何在 c++ 中读取二进制文件中的小端整数

How to read little endian integers in binary files in c++

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