NETworking with UDP - networking

Question

what happens when a UDP packet is sent to a host using sendto().
all the bits i am sending are sent(known from the return value).immediately i use a recvfrom() which does not output anything but program do not exit(i.e no return value).
I think that the program must exit if no reply is recieved.
what will the replies for a UDP packet from a port.
is this packet blocked by firewall?? if yes then why is the return value of sendto is non-negative.

Answer 1

recvfrom() will block until a message is received unless you set the socket to non-blocking.
The interfaces you want to look for are
ioctl() with FIONBIO or O_NONBLOCK (depending your platform),
select() to wait for data to arrive, or timeout after a while
Also remember that the address and port number for sendto() usually be network-byte order, so look into ntohl and ntohs.

Answer 2

you must have some error in your client or server. try localhost first, so you avoid firewall problems
this is an example of nonblocking udp client/server I was using for my tests, it uses ioctl() to check if there is data to read on the socket, however if you want to do some serious application using epoll would be more efficient also you can specify timeout to wait in microseconds:
[null#localhost tests]$ cat udpserv.c
#include <stdlib.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <errno.h>
#define BUFLEN 512
#define NPACK 15
#define PORT 9930
void diep(char *s)
{
printf("erno=%d errstr=%s\n",errno,strerror(errno));
perror(s);
exit(1);
}
int main(void)
{
struct sockaddr_in si_me, si_other;
int s,ret,nbytes, i, slen=sizeof(si_other);
char buf[BUFLEN];
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
diep("socket");
memset((char *) &si_me, 0, sizeof(si_me));
si_me.sin_family = AF_INET;
si_me.sin_port = htons(PORT);
si_me.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(s, (struct sockaddr*) &si_me, sizeof(si_me))==-1)
diep("bind");
fcntl(s, F_SETFL, fcntl(s, F_GETFL, 0) | O_NONBLOCK);
sleep(10);
for (i=0; i<NPACK; i++) {
ret=ioctl(s,FIONREAD,&nbytes);
if (ret==-1) {
printf("error on FIONREAD\n");
} else {
printf("nbytes=%d\n",nbytes);
}
if (recvfrom(s, buf, BUFLEN, 0, (struct sockaddr*) &si_other, &slen)==-1)
diep("recvfrom()");
printf("Received first half of packet from %s:%d\nData: %s\n\n",
inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port), buf);
}
close(s);
return 0;
}
[null#localhost tests]$ cat udpclient.c
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#define SRV_IP "127.0.0.1"
#define BUFLEN 200
#define NPACK 10
#define PORT 9930
/* diep(), #includes and #defines like in the server */
void diep(char *s)
{
perror(s);
exit(1);
}
int main(void)
{
struct sockaddr_in si_other;
int s, i, slen=sizeof(si_other);
char buf[BUFLEN];
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
diep("socket");
memset((char *) &si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);
if (inet_aton(SRV_IP, &si_other.sin_addr)==0) {
fprintf(stderr, "inet_aton() failed\n");
exit(1);
}
for (i=0; i<NPACK; i++) {
printf("Sending packet %d\n", i);
sprintf(buf, "This is packet %d\n", i);
if (sendto(s, buf, BUFLEN, 0, (struct sockaddr*) &si_other, slen)==-1)
diep("sendto()");
}
close(s);
return 0;
}
the sendto() is non negative because it returns number of bytes sent. check the man page for sendto