使用DnsGetCacheDataTable时内存泄漏

Question

下面的代码显示DNS客户端中缓存的域名。当内存泄漏发生在int stat = DnsGetCacheDataTable(pEntry);行时，有人能帮我找到它吗？

PS:编译代码时请使用DNSAPI.lib。

#include "stdafx.h"
#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <WinDNS.h>
#include <stdarg.h>
typedef struct _DNS_CACHE_ENTRY {
    struct _DNS_CACHE_ENTRY* pNext; // Pointer to next entry
    PWSTR pszName; // DNS Record Name
    unsigned short wType; // DNS Record Type
    unsigned short wDataLength; // Not referenced
    unsigned long dwFlags; // DNS Record FlagsB
} DNSCACHEENTRY, *PDNSCACHEENTRY;

typedef int(WINAPI *DNS_GET_CACHE_DATA_TABLE)(PDNSCACHEENTRY);
void UpdateDNS(void)
{

    PDNSCACHEENTRY pEntry = (PDNSCACHEENTRY) malloc(sizeof(DNSCACHEENTRY));
    // Loading DLL
    HINSTANCE hLib = LoadLibrary(TEXT("DNSAPI.dll"));
    // Get function address
    DNS_GET_CACHE_DATA_TABLE DnsGetCacheDataTable = (DNS_GET_CACHE_DATA_TABLE) GetProcAddress(hLib, "DnsGetCacheDataTable");
    int stat = DnsGetCacheDataTable(pEntry);
    printf("stat = %d\n", stat);
    pEntry = pEntry->pNext;
    while (pEntry) {
        wprintf(L"%s : %d \n", (pEntry->pszName), (pEntry->wType));
        pEntry = pEntry->pNext;
    }
    free(pEntry);
}

int main(int argc, char **argv) {

    while (TRUE)
    {
        Sleep(100);
        UpdateDNS();
    }   
    return 0;
}

Answer 1

尝试使用Deleaker，因为乍一看，代码看起来很好：

​

然后开始调试..。当然还有！您释放的不是原始的pEntry，而是修改的一个。

在这里，更正的代码：

void UpdateDNS(void)
{

    PDNSCACHEENTRY pEntry = (PDNSCACHEENTRY) malloc(sizeof(DNSCACHEENTRY) + 0x10000);
    PDNSCACHEENTRY pFirstEntry = pEntry;
    // Loading DLL
    HINSTANCE hLib = LoadLibrary(TEXT("DNSAPI.dll"));
    // Get function address
    DNS_GET_CACHE_DATA_TABLE DnsGetCacheDataTable = (DNS_GET_CACHE_DATA_TABLE) GetProcAddress(hLib, "DnsGetCacheDataTable");
    int stat = DnsGetCacheDataTable(pEntry);
    printf("stat = %d\n", stat);
    pEntry = pEntry->pNext;
    while (pEntry) {
        wprintf(L"%s : %d \n", (pEntry->pszName), (pEntry->wType));
        pEntry = pEntry->pNext;
    }
    free(pFirstEntry);
}

UPDATE:实际上，您不需要分配任何内存，因为DnsGetCacheDataTable本身就分配了它。试图使用DnsRecordListFree释放内存，但似乎不起作用。仍有泄漏：

​

​

最后，我得到了不会泄漏的代码：

typedef int(WINAPI *DNS_GET_CACHE_DATA_TABLE)(PDNSCACHEENTRY*);

typedef void (WINAPI *P_DnsApiFree)(PVOID pData);

void UpdateDNS(void)
{
    PDNSCACHEENTRY pEntry = NULL;
    // Loading DLL
    HINSTANCE hLib = LoadLibrary(TEXT("DNSAPI.dll"));
    // Get function address
    DNS_GET_CACHE_DATA_TABLE DnsGetCacheDataTable = (DNS_GET_CACHE_DATA_TABLE)GetProcAddress(hLib, "DnsGetCacheDataTable");
    P_DnsApiFree pDnsApiFree = (P_DnsApiFree)GetProcAddress(hLib, "DnsApiFree");
    int stat = DnsGetCacheDataTable(&pEntry);
    PVOID pFirstEntry = pEntry;
    printf("stat = %d\n", stat);
    pEntry = pEntry->pNext;
    while (pEntry) {
        wprintf(L"%s : %d \n", (pEntry->pszName), (pEntry->wType));
        pDnsApiFree(pEntry->pszName);
        PVOID p = pEntry;
        pEntry = pEntry->pNext;
        pDnsApiFree(p);
    }
}

Answer 2

这个代码有几个问题。

假设您在开始时调用LoadLibrary，而不是在结束时调用FreeLibrary。虽然技术上不是内存泄漏，但这可能不是最聪明的想法.

考虑一下，通过在循环之前直接移动到pEntry->pNext，您将跳过一个条目。当您对malloc返回的值赋值时，内存泄漏发生在同一代码行中。

PDNSCACHEENTRY pEntry = (PDNSCACHEENTRY) malloc(sizeof(DNSCACHEENTRY));
/* ... */
pEntry = pEntry->pNext;

您甚至不需要malloc，但更糟糕的是，您应该只返回malloc返回的free值，这样做是错误的：

free(pEntry);

事实上，您不仅不需要malloc (或free)来实现这一点，而且您需要的实际上是DnsRecordListFree。

下面是您的代码应该是什么样子：

PDNS_RECORD entry;
HINSTANCE hLib = LoadLibrary(TEXT("DNSAPI.dll"));
DNS_GET_CACHE_DATA_TABLE DnsGetCacheDataTable = (DNS_GET_CACHE_DATA_TABLE) GetProcAddress(hLib, "DnsGetCacheDataTable");
int stat = DnsGetCacheDataTable(&entry);
printf("stat = %d\n", stat);
for (DNSCACHEENTRY *pTemp = &entry; pTemp; pTemp = pTemp->pNext) {
    wprintf(L"%s : %d \n", pTemp->pszName, pTemp->wType);
}
DnsRecordListFree(entry, DnsFreeRecordList);