使用 DnsGetCacheDataTable 时内存泄漏

Question

以下代码展示了DNS客户端缓存的域名。有人可以帮我找到内存泄漏时的 int stat = DnsGetCacheDataTable(pEntry); 行吗？
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

这段代码有几个问题。

考虑一下您在开始时调用了 LoadLibrary 而没有在结束时调用 FreeLibrary。虽然从技术上讲不是内存泄漏，但它可能不是最聪明的主意...

考虑一下，在循环之前直接移动到 pEntry->pNext ，您将跳过一个条目。当您分配 malloc:

返回的值时，您的内存泄漏发生在同一行代码中
PDNSCACHEENTRY pEntry = (PDNSCACHEENTRY) malloc(sizeof(DNSCACHEENTRY)); /* ... */ pEntry = pEntry->pNext;

为此您甚至不需要 malloc，但更糟糕的是，您应该只 free 值 malloc returns，呈现此错误:

free(pEntry);

事实上，你不仅不需要malloc（或free），实际上你需要的是DnsRecordListFree。

您的代码可能如下所示：

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);

Answer 2

尝试使用 Deleaker，因为乍一看代码看起来不错：

然后开始调试...当然！您释放的不是原始 pEntry，而是修改后的 pEntry。

这里是更正后的代码：

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);
}

更新：实际上您不需要分配任何内存，因为 DnsGetCacheDataTable 会自行分配内存。尝试使用 DnsRecordListFree 释放内存，但它似乎不起作用。仍然泄漏：

终于得到不泄露的代码：

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);
    }
}

使用 DnsGetCacheDataTable 时内存泄漏

Memory leak when using DnsGetCacheDataTable

c

windows

dns

memory-leaks