使用Winsock开发SMTP C程序时触发段错误求助

核心问题：空指针解引用导致段错误 #

在makeSocket函数中，你传入的result和ptr是值传递的结构体指针，而非指针的指针。这意味着函数内部对ptr = result的赋值只会修改函数内的局部变量，不会同步到main函数中的ptr变量。当main调用connSocket时，ptr仍然是初始的NULL，此时访问ptr->ai_addr就会触发空指针解引用，直接导致段错误。

修复步骤 #

1. 修改makeSocket的参数传递方式 #

将result和ptr改为指针的指针，让函数能修改main中的变量：

• 函数声明改为：

  int makeSocket(int *, struct addrinfo *, struct addrinfo **, struct addrinfo **, SOCKET *);
  

• main中调用时传地址：

  makeSocket(&iResult, &hints, &result, &ptr, &ConnectSocket);
  

• 函数内部逻辑修改：

  int makeSocket(int *iResult, struct addrinfo *hints, struct addrinfo **result, struct addrinfo **ptr, SOCKET *ConnectSocket){
      // Resolve the server address and port
      *iResult = getaddrinfo("52.97.26.134", DEFAULT_PORT, hints, result);
      if (*iResult != 0) {
          printf("getaddrinfo failed: %d\n", *iResult);
          WSACleanup();
          return 1;
      }
  
      // Attempt to connect to the first address returned by
      // the call to getaddrinfo
      *ptr = *result;
  
      // Create a SOCKET for connecting to server
      *ConnectSocket = socket((*ptr)->ai_family, (*ptr)->ai_socktype, (*ptr)->ai_protocol);
  
      if (*ConnectSocket == INVALID_SOCKET) {
          printf("Error at socket(): %d\n", WSAGetLastError());
          freeaddrinfo(*result);
          WSACleanup();
          return 1;
      }
      return 0;
  }
  

2. 修复clientReceive中的判断错误 #

函数中直接用iResult比较，而不是解引用后的*iResult，会导致逻辑错误：

int clientReceive(int *iResult, SOCKET *ConnectSocket, char *out){
    *iResult = recv(*ConnectSocket, out, 200, 0);
    if (*iResult > 0) {
        printf("Bytes received: %d\n", *iResult);
        return 0;
    } else if (*iResult == 0) {
        printf("Connection closed\n");
        return 1;
    } else {
        printf("recv failed: %d\n", WSAGetLastError());
        return 2;
    }

    return -1;
}

3. 避免重复调用WSACleanup #

closeSocketListener和closeSocket都调用了WSACleanup，但该函数只需要调用一次，建议只在closeSocket中保留：

int closeSocketListener(int *iResult, SOCKET *ConnectSocket){
    // shutdown the send half of the connection since no more data will be sent
    *iResult = shutdown(*ConnectSocket, SD_SEND);
    if (*iResult == SOCKET_ERROR) {
        printf("shutdown failed: %d\n", WSAGetLastError());
        closesocket(*ConnectSocket);
        return 1;
    }
    return 0;
}

修复后流程说明 #

修改后，makeSocket会将getaddrinfo返回的地址链表头指针同步到main的result和ptr中，connSocket就能正确访问ptr指向的地址信息，彻底避免空指针错误。

内容的提问来源于stack exchange，提问作者fpp