报错: “”有指定的 C 链接，但返回了与 C 不兼容的 UDT“”?

#define DLL_EXPORT __declspec(dllexport)
extern "C" DLL_EXPORT ReturnTypeProcessMulti  processWithPatientMulti(char* storeUrl, Monitor monitor);

struct ReturnTypeProcessMulti {
    int code;
    char* msg;
    Datum obj[400];
    int count=0;
    Patient patient;
};

运行出错：2>E:\Qtwork\DeviceDll\DeviceDll\include\Interface\LemonElfNativeInterface.h(98): warning C4190: “processWithPatientMulti”有指定的 C 链接，但返回了与 C 不兼容的 UDT“ReturnTypeProcessMulti”

struct ReturnTypeProcessMulti与C不兼容