为什么值会意外变化？ inet_ntoa

Question

我正在尝试创建一个 function 来收集机器上存在的所有 IP，并且能够检测到参数中的 IP 何时存在。

问题是包含我的 IP 的变量的值在另一个 IN_ADDR 上完成 inet_ntoa 后发生变化。

我尝试使用 std::string 没有成功。 我知道 inet_ntoa 已被弃用，但我尝试了其他方法但没有成功。 我也想了解我做错了什么。

void CIPUtilities::AwaitIPAddress(struct sockaddr_in WFIAddr) {
// also tried with AwaitIPAddress(char * IPName)
char* IPName = inet_ntoa(WFIAddr.sin_addr);
printf("\n Waiting for the IP:  %s to be ready.\n", IPName);
int i;
bool ctn = TRUE;

/* Variables used by GetIpAddrTable */
PMIB_IPADDRTABLE pIPAddrTable;
DWORD dwSize = 0;
DWORD dwRetVal = 0;
IN_ADDR IPAddr;
ULONG outBufLen = 0;
ULONG Iterations = 0;

/* Variables used to return error message */
LPVOID lpMsgBuf;

// Allocate a 15 KB buffer to start with.
outBufLen = WORKING_BUFFER_SIZE;
do {

    do {
        //printf("inside do\n");
        Sleep(300); //to be removed ? Only there to avaoid to loop too much.
        pIPAddrTable = (MIB_IPADDRTABLE*)MALLOC(outBufLen);

        if (pIPAddrTable == NULL) {
            printf("Memory allocation failed for GetIpAddrTable\n");
            //exit(1); // no need to exit we need debug
        }

        dwRetVal = GetIpAddrTable(pIPAddrTable, &outBufLen, 0);

        if (dwRetVal == ERROR_BUFFER_OVERFLOW) {
            FREE(pIPAddrTable);
            pIPAddrTable = NULL;
        }
        else {
            break;
        }
    } while ((dwRetVal == ERROR_BUFFER_OVERFLOW) && (Iterations < MAX_TRIES));

    if (dwRetVal == NO_ERROR) {
        // If successful, search the IP from the data we retrived
        for (i = 0; i < (int)pIPAddrTable->dwNumEntries; i++) {
            IPAddr.S_un.S_addr = (u_long)pIPAddrTable->table[i].dwAddr;
            printf("1- The value of IPName is %s\n", IPName);
            printf("\tIP Address[%d]:     \t%s\n", i, inet_ntoa(IPAddr));
            printf("2- The value of IPName is %s\n", IPName);
            //if (strcmp(IPName, inet_ntoa(IPAddr)) == 0) {
            //    printf("IP adress[%s] is found in AddrTable\n", inet_ntoa(IPAddr));
            //    ctn = FALSE; // We exit the loop because the adress is created.
            //    break; // no need to parse more addresses.
            //}
        }
    }
    else
    {
        printf("GetIpAddrTable failed with error %d\n", dwRetVal);
        printf("Required size should be %d\n", dwSize);
        if (FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, dwRetVal, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),       // Default language
            (LPTSTR)&lpMsgBuf, 0, NULL)) {
            printf("\tError: %s", (char*)lpMsgBuf);
            LocalFree(lpMsgBuf);
        }
        //exit(1);  // no need to exit we need debug
    }

    if (pIPAddrTable) {
        FREE(pIPAddrTable);
        pIPAddrTable = NULL;
    }

} while (ctn);}

基本上 output 将是：

Waiting for the IP:  10.0.4.3 to be ready.
1- The value of IPName is 10.0.4.3
        IP Address[0]:          160.223.17.135
2- The value of IPName is 160.223.17.135
1- The value of IPName is 160.223.17.135
        IP Address[1]:          169.254.165.50
2- The value of IPName is 169.254.165.50
1- The value of IPName is 169.254.165.50
        IP Address[2]:          10.0.4.3
2- The value of IPName is 10.0.4.3
1- The value of IPName is 10.0.4.3
        IP Address[3]:          10.0.12.44
2- The value of IPName is 10.0.12.44
1- The value of IPName is 10.0.12.44
        IP Address[4]:          192.168.0.17
2- The value of IPName is 192.168.0.17
1- The value of IPName is 192.168.0.17
        IP Address[5]:          127.0.0.1
2- The value of IPName is 127.0.0.1
1- The value of IPName is 127.0.0.1

我删除了 function 的某些部分以使其更具可读性。

Answer 1

简而言之：这是因为您直接使用返回的指针而不是复制字符串。

char* IPName = inet_ntoa(WFIAddr.sin_addr);

如 inet_ntoa 文档中所述（请参阅此处的 Windows 或此处的 Linux），结果是指向static缓冲区的指针：

保证返回的字符串仅在下一次 Windows Sockets function 调用在同一线程中之前有效。 因此，应该在另一个 Windows Sockets 调用之前复制数据。

您需要做的就是将缓冲区的内容复制到一个字符串变量中。

char IPName[18];
char* temp = inet_ntoa(WFIAddr.sin_addr); 
if (temp != NULL)
{
  strcpy(IPName, temp);
}

（有关缓冲区大小的详细信息，请参见此处）

当然，您应该处理错误情况（如果 temp为NULL）