Question

I am writing a small socket program in C. In server side I create a socket descriptor using socket() system call, then I am binding that socket with a port. After this I am trying to get the IP/Port no of the descriptor, it gives port no different then the bind port no. I am trying to get back IP/Port using getsockname() method, Is it right to use this method ? Please help me.

#define SERVER_ADDR "127.0.0.1"
#define SERVER_PORT "9090" // actual port no I am binding
#define QUEUE_LENGTH 10

int main()
{
struct addrinfo hints, *servinfo, *temp;

memset(&hints,0,sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;

// here I am passing SERVER_PORT == 9090
int status = getaddrinfo(SERVER_ADDR,SERVER_PORT,&hints,&servinfo);
if(status != 0)
{
printf("Server: getaddrinfo() errored with code %d\n",status);
return;
}

int sfd = -1;
for(temp = servinfo; temp != NULL; temp = servinfo->ai_next)
{
sfd = socket(temp->ai_family,temp->ai_socktype,temp->ai_protocol);
if(sfd == -1)
{
  printf("Server: Socket error with code %d\n",sfd);
  continue;
}

status = bind(sfd,temp->ai_addr,temp->ai_addrlen);
if(status == -1)
{
  printf("Server: Bind error with code %d\n",status);
  continue;
}
printf("Server: Bind Successful\n");

// un necessary code goes here
struct sockaddr_in server_address;
char ipv4[INET_ADDRSTRLEN];
int addr_size = sizeof(server_address);
// i am using below method to get the port no from socket descriptor
getsockname(sfd, (struct sockaddr *)&server_address, &addr_size);

// I am expecting below will print 9090. but it prints different port no why ? 
printf("Server Port: %d\n",server_address.sin_port);
printf("Port from getsddrinfo: %d\n",( (struct sockaddr_in *)temp->ai_addr)->sin_port);

inet_ntop(AF_INET, &(server_address.sin_addr),ipv4,INET_ADDRSTRLEN);
printf("Server IP Address: %s\n",ipv4);
// un necessary code ends here
break;
}

if(temp == NULL)
{
printf("Server: Failed to bind\n");
return;
}
status = listen(sfd,QUEUE_LENGTH);
if(status == -1)
{
printf("Server: Listening failed\n");
return;
}
printf("Server: waiting for coneections...\n");

while(1)
{
printf("Server: Main loop, will wait for client to connect...\n");
struct sockaddr client_address;
int addr_length = sizeof client_address;
// accepting client
int new_sfd = accept(sfd,&client_address,&addr_length); 
}
printf("Server: Done!\n"); 
}

Output is:

Server: Bind Successful
Server Port: 33315 --> why this different from one I have binded (9090)
Port from getsddrinfo: 33315 --> why this different from one I have binded (9090)
Server IP Address: 127.0.0.1
Server: waiting for coneections...
Server: Main loop, will wait for client to connect...

Answer 1

The decimal members of struct sockaddr are returned in network byte order.

So you need to convert such values to host byte order, using the ntoh family of functions before using them, printing them.