我正在用 c 构建一个代理服务器，我正在尝试理解该select()功能。我已经完成了代码，以便从客户端建立连接，然后提取网址，以便可以建立另一个连接以连接到实际的 Web 服务器。然后该页面被代理接收并传回给客户端。
我知道这select()将允许它处理多个客户端请求，但我不明白它如何帮助（或者更确切地说如何实现）与 Web 服务器的第二个连接。据我了解，我将不再需要一个 while 循环来继续从 Web 服务器接收数据并将其传递回客户端。
我是否需要为 Web 服务器连接设置第二个文件描述符？如果我正在处理两个或多个客户端请求，我如何确保它们与 Web 服务器的正确连接链接在一起？无论如何，我会很感激任何帮助。我已经完成了网络教程和其他一些在线教程，但几天后我仍然没有完全理解这一点。
void error(const char *msg)
{
perror(msg);
exit(1);
}
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET)
return &(((struct sockaddr_in*)sa)->sin_addr);
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
string getHostString(const char *buf);
int main(int argc, char *argv[])
{
int sockfd, newsockfd, portno;
int fdmax; //maximum file descriptor number
socklen_t clilen;
char buffer[256];
struct sockaddr_storage remoteaddr; //client address
char clientIP[INET6_ADDRSTRLEN];
//fd_set readfds, writefds, exceptfds;
fd_set masterfds, readfds;
struct timeval timeout;
int rc;
/*Set time limit. */
timeout.tv_sec = 3;
timeout.tv_usec = 0;
/*Create a descriptor set containing the sockets */
FD_ZERO(&readfds);
FD_ZERO(&masterfds);
/*FD_ZERO(&writefds);
FD_ZERO(&exceptfds);
FD_SET(newsockfd, &readfds);
rc = select(sizeof(readfds)*8, &readfds, NULL, NULL, &timeout);
if (rc==-1){
perror("select failed");
return -1;
}*/
struct sockaddr_in serv_addr, cli_addr;
int n;
if (argc < 2) {
fprintf(stderr,"ERROR, no port provided\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0)
error("ERROR on binding");
if((listen(sockfd,5)) == -1 )
error("Server-listen() error!!!");
printf("Server-listen() is OK...\n");
FD_SET(sockfd, &masterfds);
//keep track of the biggest file descriptor so far
fdmax = sockfd; //so far it's this one
for(;;){
readfds = masterfds;
if(select(fdmax+1, &readfds, NULL, NULL, NULL) == -1){
error("Server-select() error !");
}
printf("Server-select() is OK...\n");
for(int i = 0; i <= fdmax; i++){
printf("i: %d sockfd %d\n", i, sockfd);
if(FD_ISSET(i, &readfds)){
if(i == sockfd){ //sockfd is the listener
//following handles new connections
clilen = sizeof(cli_addr);
if((newsockfd = accept(sockfd,(struct sockaddr *) &cli_addr, &clilen)) == -1)
error("Server-accept() error!!!");
else{
printf("Server-accept() is OK...\n");
FD_SET(newsockfd, &masterfds); //add to master set
if(newsockfd > fdmax)
fdmax = newsockfd;
printf("selectserver: New connection from %s on "
"socket %d\n",
inet_ntop(remoteaddr.ss_family,
get_in_addr((struct sockaddr*)&cli_addr),
clientIP, INET6_ADDRSTRLEN),
newsockfd);
}
}
else{
//handle data from a client
//Here is where the FIRST read occurs
bzero(buffer,256);
//n = read(newsockfd,buffer,255);
n = read(i,buffer,255);
if (n < 0) error("ERROR reading from socket");
printf("Here is the message: \n\n%s\n\n",buffer);
//string hoststring(buffer);
string hoststring(getHostString(buffer));
int html_port = 80;
int html_socket;
printf("Prior to struct addrinfo\n");
struct addrinfo hints, *res;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
printf("Prior to getaddrinfo()\n");
char *address = new char[hoststring.size() +1];
address[hoststring.size()] = 0;
memcpy(address, hoststring.c_str(), hoststring.size());
//getaddrinfo(token, (char *)html_port, &hints, &res);
getaddrinfo(address, "80", &hints, &res);
printf("We are past getaddrinfo()\n");
//if (html_socket = socket(PF_INET, SOCK_STREAM, 0) < 0){
if ((html_socket = socket(res->ai_family, res->ai_socktype, res->ai_protocol)) < 0){
printf("socket connection error\n");
}
printf("We are past socket()\n");
//char* address;
//address = new char[256];
//strncpy(address, "www.cs.ucr.edu", sizeof("www.cs.ucr.edu"));
printf("address: %s\n", address);
struct hostent * host;
if ((host = gethostbyname(address)) == NULL){
printf("Problem with gethostbyname()\n");
}
printf("We are past gethostbyname() and about to connect.\n");
if ( connect(html_socket, res->ai_addr, res->ai_addrlen) < 0)
printf("Unsuccessful completion of connect()");
int bytes_sent, bytes_recvd;
char recv_buff[1024];
bytes_sent = send(html_socket, buffer, 256, 0);
cout << "bytes_sent: " << bytes_sent << endl;
//do{
bytes_recvd = recv(html_socket, recv_buff, 1024, 0);
cout << "bytes_rcvd: " << bytes_recvd << endl;
cout << recv_buff << endl;
//FD_ZERO(&readfds);
//FD_SET(newsockfd, &writefds);
//n = write(newsockfd,"I got your message\n",20);
//n = write(newsockfd,recv_buff,bytes_recvd);
n = write(i,recv_buff,bytes_recvd);
if (n < 0) error("ERROR writing to socket");
bzero(recv_buff, 1024);
//}while(bytes_recvd !=0);
}
}
}
}
close(newsockfd);
close(sockfd);
return 0;
}
string getHostString(const char *buf){
string hoststring(buf);
hoststring = hoststring.substr(11);
cout << "hoststring: " << hoststring << endl;
int slashpos;
slashpos = hoststring.find("/");
int suffixendpos = hoststring.find("H") - slashpos;
string suffix = hoststring.substr(slashpos, suffixendpos);
hoststring = hoststring.substr(0, slashpos);
cout << "hoststring: " << hoststring << endl;
cout << "suffix: " << suffix << endl;
return hoststring;
}