新建头文件.h
内容如下：

#pragma once
#include<Windows.h>
extern void* g_Port;
#if 1
char* getErrorMsg(int code);
#endif

cpp

#include "global.h"
#include"pch.h"
void* g_Port = INVALID_HANDLE_VALUE;  //串口句柄



char* getErrorMsg(int code)
{
  switch (code)
  {
  case 0:
    return "成功";
  case 1:
    return "失败";
  case 2:
    return "数据非法，报文格式错";
  case 3:
    return "报文长度错误";
  case 4:
    return "HASH校验错";
  case 5:
    return "指令不存在";
  case 6:
    return "类型不存在";
  case 7:
    return "保存失败";
  default:
    break;
  }
  return "";
}

发现项目编译时出现：

WINDOWS.H already included. MFC apps must not #include <Windows.h>

百度上说windows要在mfc库下

把cpp头文件顺序改成以下即可

#include"pch.h"
#include "global.h"