C++ Sockets recv() syscall returning -1

Question

I'm currently having a problem passing messages between a server and client. As far as I know, I am properly following best practices for socket programming outlined by Beej's Socket Programming Tutorial .

When I run the two processes, the recv() syscall returns -1 (an error), rather than the number of bytes received. Also when trying to output the buf, there are a bunch of gobbledygook characters. Which makes sense, because of the error.

I'm wondering if someone could steer me in the right direction as to why I am having issues with recv()? The following are relevant code snippets.

Server:

struct sockaddr_storage their_addr;
socklen_t addr_size;
int sockfd, newfd, byte_count, status;
char buf[512];
struct addrinfo hints,  *res;

//  first,  load  up  address  structs  with  getaddrinfo():
memset(&hints,  0,  sizeof  hints);
hints.ai_family  =  PF_INET;
hints.ai_socktype  =  SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

// get address info, print stuff if error
if((status = getaddrinfo("nunki.usc.edu",  "21957",  &hints,  &res)) !=0){
    fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
    exit(1);
}

//  make  a  socket:
if((sockfd  =  socket(res->ai_family,  res->ai_socktype,  res->ai_protocol)) == -1){
    cout << "socket fail" << endl;
}

// bind the socket to the port
bind(sockfd, res->ai_addr, res->ai_addrlen);

// required output
cout << "Phase1: Login server has TCP port number " << "21957 " 
     << "and IP address " << getIPfromHost("nunki.usc.edu") << endl;

// listen for incoming connections
listen(sockfd, 10);
cout << "after listen" << endl;

// halt until receipt 
addr_size = sizeof(their_addr);
newfd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size);
cout << "after accept" << endl;

// Now  that  we're  connected,  we  can  receive  some data
byte_count  =  recv(sockfd,  buf,  sizeof  buf,  0); 
printf("recv()'d  %d  bytes  of  data  in  buf\n",  byte_count);
printf("Msg is %s\n", buf);

Client:

struct addrinfo hints,  *res;
int  sockfd;

//  first,  load  up  address  structs  with  getaddrinfo():
memset(&hints,  0,  sizeof  hints);
hints.ai_family  =  AF_INET;
hints.ai_socktype  =  SOCK_STREAM;
getaddrinfo("nunki.usc.edu",  "21957",  &hints,  &res);

//  make  a  socket:
if((sockfd  =  socket(res->ai_family,  res->ai_socktype,  res->ai_protocol)) == -1){
    cout << "socket fail" << endl;
}

// attempt connection to port
if(connect(sockfd,  res->ai_addr,  res->ai_addrlen) == -1){
    cout << "connect fail" << endl;
}

// send message to server
cout << "sockfd " << sockfd << endl;
int byte_count = send(sockfd, "Hello", 5, 0); 
cout << byte_count << endl;

The following is the output for Server:

Phase1: Login server has TCP port number 21957 and IP address 68.181.201.3
after listen
after accept
recv()'d  -1  bytes  of  data  in  buf
Msg is ÿhÿ?sÈ
Glæ

The following is the output for Client:

sockfd 4
5

Answer 1

You are calling recv on the wrong socket. You need to recv on newfd :

byte_count = recv(newfd, buf, sizeof buf, 0); /* newfd instead of sockfd. */

Now that that's out of the way,

As far as I know, I am properly following best practices for socket programming

I completely disagree.

• You are not checking return statuses for listen , bind , getaddrinfo etc
• There's not strerror or perror in your program

Answer 2

您想使用accept（）返回的套接字来接收recv（）

byte_count  =  recv(newfd,  buf,  sizeof  buf,  0); 

Answer 3

sockfd仅用于侦听客户端， newfd用于数据传输。

Answer 4

Maybe I shouldn't write this as an answer, but as a comment. Nevertheless, IMHO your use of getaddrinfo() seems wrong to me.

• On the client side, it is supposed to be called and then iterated through the results until a connection can be established.

  so

   struct addrinfo * r2 sockfd = -1; for (r2=res; r2; r2=r2->ai_next) { // make a socket: if((sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol)) == -1){ continue; // next result } // attempt connection to port if(connect(sockfd, res->ai_addr, res->ai_addrlen) == -1){ close(sockfd); sockfd = -1; continue; } } if (sockfd == -1) { // do error handling } 

  In this way, you can check all possible connections.

• On the server side, it is rather unusual to use getaddrinfo() . Normally, you would create an IPv6 socket and enable it to listen for IPv4 as well by using setsockopt() to unset the IPV6_V6ONLY flag. In this way, the socket listens to both IPv6 and IPv4. (Alas, not on WIndows XP AFAIK.)