'recvfrom' not working

Himalay · 04-29-2012, 06:56 AM

'recvfrom' function is not working. This function is a part of my traceroute program. 'recvfrom' doesn't return anything. The control stops here itself. Can anyone help me?
Following is the code: recv_v4.c

Code:

#include	"trace.h"

/*
 * Return: -3 on timeout
 *		   -2 on ICMP time exceeded in transit (caller keeps going)
 *		   -1 on ICMP port unreachable (caller is done)
 *		 >= 0 return value is some other ICMP unreachable code
 */
int recv_v4(int seq, struct timeval *tv)
{
	int				hlen1, hlen2, icmplen;
	socklen_t		len;
	ssize_t			n;
	struct ip		*ip, *hip;
	struct icmp		*icmp;
	struct udphdr	*udp;

	alarm(3);
	for ( ; ; ) {
		len = pr->salen;
		n = recvfrom(recvfd, recvbuf, sizeof(recvbuf), 0, pr->sarecv, &len);
		if (n < 0) {
			if (errno == EINTR)
				return(-3);		/* alarm expired */
			else
			{
				printf("\nrecvfrom error");
				exit(1);
			}
		}
		gettimeofday(tv, NULL);		/* get time of packet arrival */

		ip = (struct ip *) recvbuf;	/* start of IP header */
		hlen1 = ip->ip_hl << 2;		/* length of IP header */
	
		icmp = (struct icmp *) (recvbuf + hlen1); /* start of ICMP header */
		if ( (icmplen = n - hlen1) < 8)
		{
			printf("\nicmplen (%d) < 8", icmplen);
			exit(1);
		}
	
		if (icmp->icmp_type == ICMP_TIMXCEED &&
			icmp->icmp_code == ICMP_TIMXCEED_INTRANS) {
			if (icmplen < 8 + 20 + 8)
			{
				printf("\nicmplen (%d) < 8 + 20 + 8", icmplen);
				exit(1);
			}
			hip = (struct ip *) (recvbuf + hlen1 + 8);
			hlen2 = hip->ip_hl << 2;
			udp = (struct udphdr *) (recvbuf + hlen1 + 8 + hlen2);
 			if (hip->ip_p == IPPROTO_UDP &&
				udp->uh_sport == htons(sport) &&
				udp->uh_dport == htons(dport + seq))
				return(-2);		/* we hit an intermediate router */

		} else if (icmp->icmp_type == ICMP_UNREACH) {
			if (icmplen < 8 + 20 + 8)
			{
				printf("icmplen (%d) < 8 + 20 + 8", icmplen);
				exit(1);
			}

			hip = (struct ip *) (recvbuf + hlen1 + 8);
			hlen2 = hip->ip_hl << 2;
			udp = (struct udphdr *) (recvbuf + hlen1 + 8 + hlen2);
 			if (hip->ip_p == IPPROTO_UDP &&
				udp->uh_sport == htons(sport) &&
				udp->uh_dport == htons(dport + seq)) {
				if (icmp->icmp_code == ICMP_UNREACH_PORT)
					return(-1);	/* have reached destination */
				else
					return(icmp->icmp_code);	/* 0, 1, 2, ... */
			}
		} else if (verbose) {
			printf(" (from %s: type = %d, code = %d)\n",
					Sock_ntop_host(pr->sarecv, pr->salen),
					icmp->icmp_type, icmp->icmp_code);
		}
		/* Some other ICMP error, recvfrom() again */
	}
}

dwhitney67 · 04-29-2012, 07:19 AM

Quote:

Originally Posted by Himalay

'recvfrom' function is not working. This function is a part of my traceroute program. 'recvfrom' doesn't return anything. The control stops here itself. Can anyone help me?

If your socket is set up using the default settings, then it is blocking socket. Thus recvfrom() could potentially block indefinitely. When the alarm() expires, recvfrom() will return a result value of less than 0; errno will be set to EINTR. So when you indicate that recvfrom() is not returning anything, can you please elaborate more?

Perhaps your recvfrom() is not receiving data? Are you sure that the socket is being sent any data?

P.S. In lieu of using alarm(), you should probably do something like this:

Code:

struct timeval timeout = { 3, 0};   /* 3 seconds, 0 microseconds */
setsockopt(recvfd, SOL_SOCKET, SO_RCVTIMEO, (const void*) &timeout, sizeof(timeout));

Himalay · 04-30-2012, 02:33 AM

I tried your suggestion "dwhitney67" still no result.
What i mean to say control stops there itself is, suppose if i give 'printf' statement after recvfrom function 'printf' is not printing. But it is timing out i.e. sig_alrm function is called, still control doesn't come to 'if (n < 0)' statement.

dwhitney67 · 04-30-2012, 05:32 AM

Quote:

Originally Posted by Himalay

I tried your suggestion "dwhitney67" still no result.

Do you have a signal handler associated with SIGALRM? Or is your program merely terminating with the following message:

Code:

Alarm clock

Try this simple program to see if you get the desired results you seek:

Code:

#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>

int main()
{
    char buf[50];

    int sd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

    /* typically the socket would be bound to a port; we don't care about this for this example. */

    struct timeval timeout = { 3, 0};   /* 3 seconds, 0 microseconds */
    setsockopt(sd, SOL_SOCKET, SO_RCVTIMEO, (char*) &timeout, sizeof(timeout));

    int n = recvfrom(sd, buf, sizeof(buf), 0, NULL, NULL);

    if (n < 0)
    {
        if (errno == EINTR)
        {
            fprintf(stderr, "recvfrom() interrupted.\n");
        }
        else
        {
            fprintf(stderr, "Error from recvfrom() - %s (%d)\n", strerror(errno), errno);
        }
    }

    return 0;
}

If copied correctly, the program above will produce this result:

Code:

Error from recvfrom() - Resource temporarily unavailable (11)

The signal, 11, is equivalent to EAGAIN (or EWOULDBLOCK); you can take this to imply that a timeout occurred.

Himalay · 04-30-2012, 05:43 AM

I tried your code 'dwhitney', nothing is being printed. It is hanging it seems.
Yes. I am having signal handler. It prints 'inside signal' and then it is hanging it seems.
Following is the code.

Code:

#include        "trace.h"

int gotalarm;

void
sig_alrm(int signo)
{
printf("\n inside signal ");
        gotalarm = 1;   /* set flag to note that alarm occurred */
        return;                 /* and interrupt the recvfrom() */
}

dwhitney67 · 04-30-2012, 06:08 AM

Quote:

Originally Posted by Himalay

I tried your code 'dwhitney', nothing is being printed. It is hanging it seems.

Here are some basic questions:
1) What OS are you using?
2) What compiler are you using?
3) What steps did you take to build the sample application I offered?

Also, a suggestion for your application... when using printf(), output a newline character at the end of the text you wish to see; this forces a flush of the output stream. Otherwise the output you are hoping to see is buffered until some byte limit is reached (1K bytes, 4K bytes, ??).

Himalay · 04-30-2012, 06:28 AM

Quote:

Here are some basic questions:
1) What OS are you using?
2) What compiler are you using?
3) What steps did you take to build the sample application I offered?

I am using Windows XP Professional OS,
CYGWIN gcc compiler
gcc test.c