VC++玩转Native Wifi API 3---GOTO在释放资源时的作用

GOTO语句有着很臭的名声，我们的老师经常教导我们说，不要轻易使用它。

C++跳转语句有三个：goto、break和continue。它们只是工具，我觉得问题不能归咎于工具，问题在于人。

就像指针一样，goto这个无条件跳转语句力量还是很强大的，如果滥用，出现问题很难排查。

但有些时候goto确实是不二选择，例如我遇到的，在函数中有多个出口，而每个出口都遇到释放资源的时候，与其都把释放语句不厌其烦的写一遍，

不如一个goto语句来的干脆利落。

下面的例子取自上一篇Native Wifi API文章，由于我们的程序经常控制的wifi的on和off，必须注意释放资源。就拿WlanOpenHandle来说，

如果不注意对称WlanCloseHandler，程序几次运行后报错：ERROR_REMOTE_SESSION_LIMIT_EXCEEDED

官网解释为：Too many handles have been issued by the server.

所以我们会在每个API调用后，确认返回值，如果错误，程序将不再继续向下运行，return之前，我们必须释放资源。当出口很多时，我们要写很多同样的代码，

很烦躁，难读，代码急速膨胀。但使用goto后，问题便轻松了许多，请看简单例子：

// ManageWirelessNetwork.cpp : Defines the entry point for the console application.//#include "stdafx.h"#include <stdio.h>#include <windows.h>#include <shellapi.h>#include <wlanapi.h>// Need to link with shell32.lib#pragma comment(lib, "shell32.lib")#pragma comment(lib, "wlanapi.lib")int _tmain(int argc, _TCHAR* argv[]){    DWORD dwResult = 0;    DWORD dwMaxClient = 2;    DWORD dwCurVersion = 0;    HANDLE hClient = NULL;    PWLAN_INTERFACE_INFO_LIST pIfList = NULL;    PWLAN_INTERFACE_INFO pIfInfo = NULL;    dwResult = WlanOpenHandle(dwMaxClient, NULL, &dwCurVersion, &hClient);    if (dwResult != ERROR_SUCCESS) {        wprintf(L"WlanOpenHandle failed with error: %u\n", dwResult);        return false;    }    dwResult = WlanEnumInterfaces(hClient, NULL, &pIfList);    if (dwResult != ERROR_SUCCESS) {wprintf(L"WlanEnumInterfaces failed with error: %u\n", dwResult);goto RELEASE_RESOURCE;    }        WLAN_PHY_RADIO_STATE state;    state.dwPhyIndex = 0;    state.dot11SoftwareRadioState = dot11_radio_state_on;//off here too.    PVOID pData = &state;    dwResult = WlanSetInterface(hClient,&pIfList->InterfaceInfo[0].InterfaceGuid,        wlan_intf_opcode_radio_state,sizeof(WLAN_PHY_RADIO_STATE),pData,NULL);    if(dwResult == ERROR_SUCCESS)    {        wprintf(L"set state success!\n");    }    else    {        wprintf(L"set state failed!err is %d\n",dwResult);    }RELEASE_RESOURCE:if(hClient){WlanCloseHandle(hClient,NULL);hClient = NULL;}if(pIfList){WlanFreeMemory(pIfList);pIfList = NULL;}if(pIfInfo){WlanFreeMemory(pIfInfo);pIfInfo = NULL;}return 0;}

最后，goto还会用来跳出多重循环。但需要注意的是，只能从内层跳到外层，不可逆操作。