VC++玩轉Native Wifi API 3---GOTO在釋放資源時的作用

VC++玩轉Native Wifi API 3---GOTO在釋放資源時的作用

GOTO語句有著很臭的名聲，我們的老師經常教導我們說，不要輕易使用它。

C++跳轉語句有三個：goto、break和continue。它們只是工具，我覺得問題不能歸咎於工具，問題在於人。

就像指針一樣，goto這個無條件跳轉語句力量還是很強大的，如果濫用，出現問題很難排查。

但有些時候goto確實是不二選擇，例如我遇到的，在函數中有多個出口，而每個出口都遇到釋放資源的時候，與其都把釋放語句不厭其煩的寫一遍，

不如一個goto語句來的干脆利落。

下面的例子取自上一篇Native Wifi API文章，由於我們的程序經常控制的wifi的on和off，必須注意釋放資源。就拿WlanOpenHandle來說，

如果不注意對稱WlanCloseHandler，程序幾次運行後報錯：ERROR_REMOTE_SESSION_LIMIT_EXCEEDED

官網解釋為：Too many handles have been issued by the server.

所以我們會在每個API調用後，確認返回值，如果錯誤，程序將不再繼續向下運行，return之前，我們必須釋放資源。當出口很多時，我們要寫很多同樣的代碼，

很煩躁，難讀，代碼急速膨脹。但使用goto後，問題便輕松了許多，請看簡單例子：

// ManageWirelessNetwork.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include 
#include 
#include 
#include 

// 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;
}