What typedef BOOL (WINAPI *FN_SetupDiGetDeviceProperty)?

我在这里找到了一些代码，列出了计算机上的所有USB设备。代码如下：

#include <windows.h>
#include <devguid.h>    // for GUID_DEVCLASS_CDROM etc
#include <setupapi.h>
#include <cfgmgr32.h>   // for MAX_DEVICE_ID_LEN, CM_Get_Parent and CM_Get_Device_ID
#define INITGUID
#include "c:WinDDK7600.16385.1incapidevpkey.h"
#include <tchar.h>
#include <stdio.h>
#include <iostream>
#define ARRAY_SIZE(arr)     (sizeof(arr)/sizeof(arr[0]))
#pragma comment (lib, "setupapi.lib")
typedef BOOL (WINAPI *FN_SetupDiGetDeviceProperty)(
  __in       HDEVINFO DeviceInfoSet,
  __in       PSP_DEVINFO_DATA DeviceInfoData,
  __in       const DEVPROPKEY *PropertyKey,
  __out      DEVPROPTYPE *PropertyType,
  __out_opt  PBYTE PropertyBuffer,
  __in       DWORD PropertyBufferSize,
  __out_opt  PDWORD RequiredSize,
  __in       DWORD Flags
);
// List all USB devices with some additional information
void ListUsbDevices(void)
{
    unsigned i, j;
    DWORD dwSize, dwPropertyRegDataType;
    DEVPROPTYPE ulPropertyType;
    OSVERSIONINFO osvi;
    CONFIGRET r;
    HDEVINFO hDevInfo;
    SP_DEVINFO_DATA DeviceInfoData;
    const static LPCTSTR arPrefix[3] = {TEXT("VID_"), TEXT("PID_"), TEXT("MI_")};
    TCHAR szDeviceInstanceID [MAX_DEVICE_ID_LEN];
    TCHAR szDesc[1024];
    LPTSTR pszToken, pszNextToken;
    TCHAR szVid[MAX_DEVICE_ID_LEN], szPid[MAX_DEVICE_ID_LEN], szMi[MAX_DEVICE_ID_LEN];
#ifdef UNICODE
    FN_SetupDiGetDeviceProperty fn_SetupDiGetDeviceProperty = (FN_SetupDiGetDeviceProperty)
        GetProcAddress (GetModuleHandle (TEXT("Setupapi.dll")), "SetupDiGetDevicePropertyW");
#else
    FN_SetupDiGetDeviceProperty fn_SetupDiGetDeviceProperty = (FN_SetupDiGetDeviceProperty)
        GetProcAddress(GetModuleHandle(TEXT("Setupapi.dll")), "SetupDiGetDevicePropertyA");
#endif
    // List all connected USB devices
    hDevInfo = SetupDiGetClassDevs (NULL, TEXT("USB"), NULL, DIGCF_PRESENT|DIGCF_ALLCLASSES);
    if (hDevInfo == INVALID_HANDLE_VALUE)
        return;
    // Find the ones that are driverless
    for (i = 0; ; i++)  {
        DeviceInfoData.cbSize = sizeof (DeviceInfoData);
        if (!SetupDiEnumDeviceInfo(hDevInfo, i, &DeviceInfoData))
            break;
        r = CM_Get_Device_ID(DeviceInfoData.DevInst, szDeviceInstanceID , MAX_PATH, 0);
        if (r != CR_SUCCESS)
            continue;
        _tprintf (TEXT("%sn"), szDeviceInstanceID );
        if (SetupDiGetDeviceRegistryProperty (hDevInfo, &DeviceInfoData, SPDRP_DEVICEDESC,
                                              &dwPropertyRegDataType, (BYTE*)szDesc,
                                              sizeof(szDesc),   // The size, in bytes
                                              &dwSize))
            _tprintf (TEXT("    Device Description: "%s"n"), szDesc);
        // Retreive the device description as reported by the device itself
        memset(&osvi, 0, sizeof(OSVERSIONINFO));
        osvi.dwOSVersionInfoSize = sizeof(OSVERSIONINFO);
        if ( (GetVersionEx(&osvi) != 0)
            && (osvi.dwBuildNumber >= 7000) ) {
                // On Vista and earlier, we can use only SPDRP_DEVICEDESC
                // On Windows 7, the information we want ("Bus reported device description") is
                // accessed through DEVPKEY_Device_BusReportedDeviceDesc
                if (fn_SetupDiGetDeviceProperty && fn_SetupDiGetDeviceProperty (hDevInfo, &DeviceInfoData, &DEVPKEY_Device_BusReportedDeviceDesc,
                                                                                &ulPropertyType, (BYTE*)szDesc, sizeof(szDesc), &dwSize, 0))
                    _tprintf (TEXT("    Bus Reported Device Description: "%s"n"), szDesc);
        }
        pszToken = _tcstok_s (szDeviceInstanceID , TEXT("\#&"), &pszNextToken);
        while(pszToken != NULL)
        {
            szVid[0] = TEXT('');
            szPid[0] = TEXT('');
            szMi[0] = TEXT('');
            for (j = 0; j < 3; j++) {
                if (_tcsncmp(pszToken, arPrefix[j], lstrlen(arPrefix[j])) == 0) {
                    switch(j) {
                        case 0:
                            _tcscpy_s(szVid, ARRAY_SIZE(szVid), pszToken);
                            break;
                        case 1:
                            _tcscpy_s(szPid, ARRAY_SIZE(szPid), pszToken);
                            break;
                        case 2:
                            _tcscpy_s(szMi, ARRAY_SIZE(szMi), pszToken);
                            break;
                        default:
                            break;
                    }
                }
            }
            if (szVid[0] != TEXT(''))
                _tprintf (TEXT("    vid: "%s"n"), szVid);
            if (szPid[0] != TEXT(''))
                _tprintf (TEXT("    pid: "%s"n"), szPid);
            if (szMi[0] != TEXT(''))
                _tprintf (TEXT("    mi: "%s"n"), szMi);
            pszToken = _tcstok_s (NULL, TEXT("\#&"), &pszNextToken);
            //VID and PID for the playstation Move.
            if( szVid[4] == TEXT('8') && 
                szVid[5] == TEXT('8') && 
                szVid[6] == TEXT('8') && 
                szVid[7] == TEXT('8') &&
                szPid[4] == TEXT('0') &&
                szPid[5] == TEXT('3') &&
                szPid[6] == TEXT('0') &&
                szPid[7] == TEXT('8'))
            {
                std::cout << "PlayStation Move Detected!" << std::endl;
                break;
            }
        }
    }
    return;
}
int main()
{
    int iQuit;
    ListUsbDevices();
    std::cin >> iQuit;
    return 0;
}

为什么我们需要先定义FN_SetupDiGetDeviceProperty代码块？它到底是做什么的？我认为SetupDiGetDeviceProperty函数是WDK的一部分，因此我们可以立即使用它？