Error connecting to server by proxy server in c++ socket programming - c++

Question

The code for connection :
cout << "connecting1\n";
WSADATA wsadata;
int iResult = WSAStartup (MAKEWORD(2,2), &wsadata );
if (iResult !=NO_ERROR )
printf("\nmyERROR at WSAStartup()\n");
int sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock == -1) {
perror("error opening socket"); return -1;
}
struct sockaddr_in sin;
sin.sin_port = htons(port);
sin.sin_addr.s_addr = inet_addr(host.c_str());
sin.sin_family = AF_INET;
if (connect(sock, (struct sockaddr *)&sin, sizeof(sin)) == -1) {
perror("error connecting to host"); return -1;
}
const int query_len = query.length() + 1; // trailing '\0'
if (send(sock, query.c_str(), query_len, 0) != query_len) {
perror("error sending query"); return -1;
}
const int buf_size = 1024 * 1024;
while (true) {
std::vector<char> buf(buf_size, '\0');
const int recv_len = recv(sock, &buf[0], buf_size - 1, 0);
if (recv_len == -1) {
perror("error receiving response"); return -1;
} else if (recv_len == 0) {
std::cout << std::endl; break;
} else {
std::cout << &buf[0];
fprintf(fp, "%s", &buf[0]);
}
}
In wifi without proxy it works fine, but when we use proxy server, net can be accessed in chrome, but the above code prints
connecting1
error connecting to host
What is the problem ?