Class 23: Homework

You must print this sheet out and write/type answers on it!

Suppose we have the following declarations:

char msg[257];
struct sockaddr_in sourcesa, targetsa;
int ssize = sizeof(sourcesa);

The usual ways to call sendto and recvfrom are:

int s = sendto(sfd,msg,strlen(msg),0,(struct sockaddr*)&targetsa,sizeof(targetsa));
int r = recvfrom(sfd,msg,256,0,(struct sockaddr*)&sourcesa,&ssize);

explain why calling recvfrom like this

recvfrom(sfd,msg,256,0,NULL,NULL);

is OK, but calling sendto like this

sendto(sfd,msg,strlen(msg),0,NULL,0);

is not. Hint: read the book and/or read the recvfrom/sendto man pages.

Explain why the TCP server described in Class 21 needed to use the listen and accept system calls, while the UDP server described below does not.
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How would you modiy the server code below to print out the port number of the client for each datagram recieved?
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Why wouldn't all the clients' port numbers be 10000?
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 1 /***********************************************************
 2  * A simple UDP server program.  It just reads messages and
 3  * prints them out along with the identity of sending host.
 4  ***********************************************************/
 5 #include <sys/types.h>
 6 #include <stdio.h>
 7 #include <stdlib.h>
 8 #include <sys/socket.h>
 9 #include <netinet/in.h>
10 #include <arpa/inet.h>
11 #include <netdb.h>
12 
13 int main(int argc, char **argv)
14 {
15   /* Set up socket */
16   int sfd = socket(AF_INET,SOCK_DGRAM,0);
17   if (sfd == -1) { fprintf(stderr,"Socket not created!\n"); exit(1); }
18 
19   /* Get current host's IP address. */
20   struct hostent *p;
21   p = gethostbyname(getenv("HOST"));
22   if (p == NULL) { fprintf(stderr,"Name not found!\n"); exit(2); }
23   unsigned int *ip = (unsigned int*)(p->h_addr_list[0]);
24 
25   /* Set up address structure */
26   struct sockaddr_in mysa;
27   mysa.sin_family = AF_INET;
28   mysa.sin_addr.s_addr = *ip;
29   mysa.sin_port = htons(10000);
30 
31   /* Bind socket to address */
32   if (bind(sfd,(struct sockaddr*)&mysa,sizeof(mysa)) < 0)
33   {
34     close(sfd);
35     fprintf(stderr,"bind failed!\n");
36     exit(3);
37   }
38 
39   while(1)
40   {
41     /* Define buffer in which to recieve client's message. */
42     char msg[257] = {'\0'};
43 
44     /* Set up structure for client's host+port and recieve datagram! */
45     struct sockaddr_in sourcesa = {0};
46     int ssize = sizeof(sourcesa);
47     int r = recvfrom(sfd,msg,256,0,(struct sockaddr*)&sourcesa,&ssize);
48 
49     /* Print host and message. */
50     struct hostent *he= gethostbyaddr((char *)&sourcesa.sin_addr.s_addr,4,AF_INET);
51     printf("Got message from '%s' (IP %u)!\n",he->h_name,sourcesa.sin_addr.s_addr);
52     if (r == 0)
53       printf("-->%i-byte message!\n",r);
54     else
55       printf("-->%i-byte message: %s",r,msg);
56   }
57 
58   fprintf(stderr,"Server shutting down!\n");
59   close(sfd);
60   return 0;
61 }