背景：先用wireshark抓数据帧，过滤自己需要的之后转换为json文件，然后使用jsoncpp工具解析，获取其中有用的数据，最后把数据写入到文件中，之后分析数据或根据数据画图。
我分析的json文件格式如下，是抓取的usb数据，我需要的是"usb.capdata"键值的数据，由于"23:80:45:80"是两个数据，并且是小端格式，这个数据应该是0823h和0845h，所以我我还需要做一些转换

{
    "_index": "packets-2024-04-23",
    "_type": "doc",
    "_score": null,
    "_source": {
      "layers": {
        "frame": {
          "frame.section_number": "1",
          "frame.interface_id": "0",
          "frame.interface_id_tree": {
            "frame.interface_name": "\\\\.\\USBPcap1",
            "frame.interface_description": "USBPcap1"
          },
          "frame.encap_type": "152",
          "frame.time": "Apr 23, 2024 17:07:55.755688000 中国标准时间",
          "frame.offset_shift": "0.000000000",
          "frame.time_epoch": "1713863275.755688000",
          "frame.time_delta": "0.000135000",
          "frame.time_delta_displayed": "0.000000000",
          "frame.time_relative": "41.255565000",
          "frame.number": "25",
          "frame.len": "31",
          "frame.cap_len": "31",
          "frame.marked": "0",
          "frame.ignored": "0",
          "frame.protocols": "usb"
        },
        "usb": {
          "usb.src": "1.12.1",
          "usb.addr": "1.12.1",
          "usb.dst": "host",
          "usb.addr": "host",
          "usb.usbpcap_header_len": "27",
          "usb.irp_id": "0xffff9503afc4da20",
          "usb.usbd_status": "0x00000000",
          "usb.function": "0x0009",
          "usb.irp_info": "0x01",
          "usb.irp_info_tree": {
            "usb.irp_info.reserved": "0x00",
            "usb.irp_info.direction": "0x01"
          },
          "usb.bus_id": "1",
          "usb.device_address": "12",
          "usb.endpoint_address": "0x81",
          "usb.endpoint_address_tree": {
            "usb.endpoint_address.direction": "1",
            "usb.endpoint_address.number": "1"
          },
          "usb.transfer_type": "0x03",
          "usb.data_len": "4",
          "usb.request_in": "23",
          "usb.time": "0.000229000",
          "usb.bInterfaceClass": "0x0a"
        },
        "usb.capdata": "23:80:45:80"
      }
    }
  }

代码如下

#include "json/json.h"
#include <fstream>
#include <iostream>
/**
 * $g++ readFromStream.cpp -ljsoncpp -std=c++11 -o readFromStream
 * $./readFromStream
 */
 void readFileJson();
int main(int argc, char* argv[]) {

  readFileJson();
  return EXIT_SUCCESS;
}

void readFileJson() {
  Json::Reader reader;
  Json::Value root;
  string ifilename, ofilename;
  cout << "please input json file name:";
  cin >> ifilename;
  cout << "please input output file name:";
  cin >> ofilename;

  cout << "start~" << endl;
  //从文件中读取，保证当前文件有json文件
  ifstream in(ifilename, ios::binary);

  if (!in.is_open()) {
    cout << "Error opening file\n";
    return;
  }

  if (reader.parse(in, root)) {

    string usb_capdata;	//usb原数据
    ofstream rf;		//数据输出句柄
    const char* c1; // string转换数组原数据
    char c2[5];     //截取翻转之后数据
    int num;        //转换为十进制
    int strsize = root.size();	//一个数据帧是一组数据，这个也表示有多少个数据帧
    rf.open(ofilename, ios::binary);	//如果没有文件则创建
    for (int i = 0; i < strsize; i++) {
      usb_capdata = root[i]["_source"]["layers"]["usb.capdata"].asString();
      c1 = usb_capdata.c_str(); //string转换为char型
      //23:80:45:80
      //第一个数据
      c2[0] = c1[3];
      c2[1] = c1[4];
      c2[2] = c1[0];
      c2[3] = c1[1];
      c2[4] = '\0';
      num = strtol(c2, NULL, 16); //转换为十进制
      rf << num << ","; //写入文件

	//第二个数据
      c2[0] = c1[9];
      c2[1] = c1[10];
      c2[2] = c1[6];
      c2[3] = c1[7];
      c2[4] = '\0';
      num = strtol(c2, NULL, 16);
      rf << num << endl;
    }
    rf.close(); //关闭数据输出文件
    cout << "Reading Complete!" << endl;
  } else {
    cout << "parse error\n" << endl;
  }
  in.close(); //关闭数据输入文件
  system("pause");
}