Question

I’m making a client program in C that has to deal with this situation:

1- server program receives udp datagram in port no 8080 sent by client with a port number X

2- server creates a new socket (TCP) in port number X

3- using this TCP socket, server reads a string sent by the client

(running on localhost)

I don’t need to make the server program, it’s already done. The points 1 and 2 are covered, but I’ve been a couple of days trying to work out the 3rd point and I’m not able to make it work ><

The code I’ve got for the client is this:

#define MYPORT 8080


int main(int argc, char *argv[ ]) {

int sockfd;

/* connector’s address information */

struct sockaddr_in their_addr;
struct hostent *he;
int numbytes;

int sockfd2, n;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[256];


if (argc != 3) {
    fprintf(stderr, "Usage: %s <hostname> <message>\n", argv[0]);
    exit(1);
}

/* get the host info */

if ((he = gethostbyname(argv[1])) == NULL) {
perror("Error obtaining the client. \n");
    exit(1);
}

else printf("Client obtained\n");

if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
    perror("Error creating UDP socket\n");
    exit(1);
}

else printf("UDP Socket done\n");

their_addr.sin_family = AF_INET;

printf("Port: 8080\n");

their_addr.sin_port = htons(MYPORT);

their_addr.sin_addr = *((struct in_addr *)he->h_addr);

memset(&(their_addr.sin_zero), '\0', 8);

 sockfd2 = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd2 < 0) 
    error("ERROR opening socket");
server = gethostbyname(argv[1]);
if (server == NULL) {
    fprintf(stderr,"ERROR, no such host\n");
    exit(0);
}

//sending port where the TCP socket will be associated
//server client connects correctly to this port 
//and the code it's working fine in this point
if((numbytes = sendto(sockfd, argv[2], strlen(argv[2]), 0, (struct sockaddr *)&their_addr, sizeof(struct sockaddr))) == -1)

{

perror("Client-sendto() error lol!");

exit(1);

}
//port is sent, now let's connect to the port by tcp and write the string
//not working properly from now on

bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr, 
     (char *)&serv_addr.sin_addr.s_addr,
     server->h_length);
serv_addr.sin_port = htons(atoi(argv[2]));
if (bind(sockfd2,(struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) 
    error("ERROR connecting");

listen(sockfd2, 5);
accept(sockfd2, 0, 0);
printf("accepted!\n");

//sending the string to the TCP Port...
if((numbytes = sendto(sockfd2, "hi", 2, 0, (struct sockaddr *)&serv_addr, sizeof(struct sockaddr))) == -1)

{

printf("Client-sendto()-TCP error\n");

exit(1);

}


if (close(sockfd) != 0) printf("Client-sockfd-UDP closing is failed!\n");
else printf("Client-sockfd-UDP successfully closed!\n");
if (close(sockfd) != 0) printf("Client-sockfd2-TCP closing is failed!\n");
else printf("Client-sockfd2-TCP successfully closed!\n");
return 0;

}

The code works for the first two steps, but in the last step, it seems it’s not connecting well with the TCP port, because my client program ends but my server program says that he receives null.
And of course I’m always sending ports > 1024

Thanks in advance, any help will be so appreciated.

Answer 1

listen(sockfd2, 5);
accept(sockfd2, 0, 0);
printf("accepted!\n");

I haven’t read all your code, but the above (at least) is wrong. You absolutely need to retain the return value of accept: it’s the socket you need to write to!

accept returns a file descriptor for the new TCP socket that has just been created for communicating with the “server” in your case. You need to use that as the file descriptor you write your string to.

(The sendto call just after that, apart from using the wrong socket, is a bit suspicious since the server will have no way to determine how much data to read/where the message stops. Passing a length of 3 (to include the \0 byte, would be a bit less suspicious.)