CODEWITHSUNDEEP
csockets
kubiej21
kubiej21

Reputation: 698

C: Issues binding an open port to a socket

I'm working on an assignment that deals with binding an available port to a TCP socket. Initially, when I wrote my server code, I set a predefined port for the system to listen on, and this worked perfectly. However, upon converting my code to bind to an open port and print the port being used, I am unable to connect to the server via telnet.

I came across another interesting anomaly when I converted my code back to using a predefined port. Once again, the server worked as expected on the predefined port, however, the port that it reported to be using was different than what I had assigned it.

This first code snippet creates a socket and uses a predefined port (32000).

/*Create TCP socket with a predefined port*/
listener = socket(AF_INET, SOCK_STREAM, 0);
if(listener == -1) {
    perror("Create socket");
    exit(1);
}/*End if*/

bzero(&servaddr, sizeof(servaddr));/*Set serraddr to 0's*/
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);/*Any of this host's interfaces is OK.*/
servaddr.sin_port = htons(32000);
bind(listener, (struct sockaddr *)&servaddr, sizeof(servaddr));
length = sizeof(servaddr);
/*getsockname(listener, (struct sockaddr *)&servaddr, &length);/*Find out port number*/
printf("RSTREAM:: assigned port number %d\n", servaddr.sin_port);

This second code snippet uses an available port. I tried setting servaddr.sin_port to 0 because I read that doing so signifies that the system is to provide an available port. I also tried commenting this line out just for the heck of it, but the results were the same.

/*Create TCP socket with an open port*/
listener = socket(AF_INET, SOCK_STREAM, 0);
if(listener == -1) {
    perror("Create socket");
    exit(1);
}/*End if*/

bzero(&servaddr, sizeof(servaddr));/*Set serraddr to 0's*/
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);/*Any of this host's interfaces is OK.*/
servaddr.sin_port = htons(0);/*bind() will provide unique port.*/
bind(listener, (struct sockaddr *)&servaddr, sizeof(servaddr));
length = sizeof(servaddr);
getsockname(listener, (struct sockaddr *)&servaddr, &length);/*Find out port number*/
printf("RSTREAM:: assigned port number %d\n", servaddr.sin_port);

Upvotes: 0

Views: 1880

Answers (1)

cnicutar
cnicutar

Reputation: 182619

You need to print the converted port number, i.e. you need to do ntohs(3):

printf("RSTREAM:: assigned port number %d\n", ntohs(servaddr.sin_port));

Upvotes: 3

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