使用DnsGetCacheDataTable时发生内存泄漏

Question

The following code shows cached domain names in DNS client. Could somebody please help me to find the memory leak when it hits int stat = DnsGetCacheDataTable(pEntry); line?
PS: please use DNSAPI.lib while compiling the code.

#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

There are several problems with this code.

Consider that you're calling LoadLibrary at the beginning without calling FreeLibrary at the end. Whilst not technically a memory leak, it's probably not the brightest idea...

Consider that by moving straight to pEntry->pNext before the loop, you're skipping an entry. Your memory leak occurs in that very same line of code, when you assign over the value returned by malloc :

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

You don't even need malloc for this, but to make matters worse, you should only ever free values that malloc returns, rendering this erroneous:

free(pEntry);

In fact, not only do you not need malloc (or free ) for this, but what do you need is actually DnsRecordListFree .

Here's what your code should probably look like:

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

Tried with Deleaker because at the first glance the code looked well:

Then started debugging... and of course! You free not original pEntry but modified one.

Here the corrected code:

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: in fact you don't need to allocate any memory because DnsGetCacheDataTable allocated it itself. Tried to free the memory using DnsRecordListFree but it seems it doesn't work. Still leaks:

Finally I got the code that doesn't leak:

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);
    }
}