I use FreeBSD7.0, I want to use UDP broadcast,my code is following:
/*udpcli01.c*/
int
main(int argc, char **argv)
{
int sockfd;
struct sockaddr_in servaddr;
if (argc != 2)
err_quit("usage: udpcli <IPaddress>");
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(13);
inet_pton(AF_INET, argv[1], &servaddr.sin_addr);
sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
dg_cli(stdin, sockfd, (SA *) &servaddr, sizeof(servaddr));
exit(0);
}

/*dg_cli.c*/
void
dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t servlen)
{
int n;
char sendline[MAXLINE], recvline[MAXLINE + 1];
while (fgets(sendline, MAXLINE, fp) != NULL) {
sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);
n = Recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL);
recvline[n] = 0; /* null terminate */
fputs(recvline, stdout);
}
}

When I run above code,10.1.2.255 is broadcast address,and input a line,like following:
$./udpcli01 10.1.2.255
Hi

It should receive many machine response,but I don't receive any message!
If I ping 10.1.2.255,I can receive many machine response message,why my code can't receive message? How can I modify above code to receive message?

Thanks in advance!

Stay away from directed broadcasts, they are dangerous. Use 255.255.255.255 instead.

Note originally 255.255.255.255 was intended to be used to broadcast to every node on the Internet. It was quickly found to be a bad idea and was changed to mean "limited broadcast" to be sent on the local link only. See RFC 1122.

Hi, you need to set SO_BROADCAST option for the socket with 'setsockopt' function.
And you need suitable server to respond...