#
/* udp_read.c */

/* EMACS_MODES: c !fill */


/* User Datagram Protocol layer for the UNIX network system.  The
 * following routines are included:
 *	udp_read	receive a udp packet
 */


#include	<udp.h>
 

udp_read (fd, buf, len)

/* Read the next available udp packet from the net into the specified
 * buffer of length len.  Return the length of the received packet in
 * bytes.  If no packet is available returns 0.
 * The macro udp_data() may be used to obtain a pointer to the start
 * of the data area of the packet.
 *
 * Arguments:
 */

int	fd;				/* file descriptor */
caddr_t	buf;				/* receive buffer */
int	len;				/* length of receive buffer */
{
	struct	udpph	ph;		/* pseudo-header */
	reg	struct	udp	*pup;	/* upd packet ptr. */
	reg	struct	ip	*pip;	/* ip packet ptr */
	int	datalen;		/* len. of rcvd. packet */
	int	inlen;			/* len. of in packet */
	int	udplen;			/* len. of udp packet */
	unshort	csum;			/* temp. checksum */
	caddr_t	pdata;			/* ptr. to data part of packet */
	
	if ((datalen = in_read (fd, buf, len)) == 0)
		return (0);
		
	pip = in_head(buf);		/* get in & udp headers */
	pup = in_data(pip);
	
	inlen = pip->ip_len;		/* get in len */
	udplen = short_to_net(pup->ud_len); /*  and udp len */
	if (udplen < sizeof (struct udp) || inlen > datalen ||
	   (udplen + (pip->ip_ihl << 2)) != inlen) /* bad length? */
		return (0);		/* yes, punt */
	
	if (udplen & 1)	{		/* if a goddam odd byte count */
		pdata = (char *)(pup);	/* must zero-pad to even count */
		pdata[udplen] = 0;
	}

	ph.ph_src = pip->ip_src;	/* build pseudo-header */
	ph.ph_dest = pip->ip_dest;
	ph.ph_zero = 0;
	ph.ph_prot = pip->ip_prot;
	ph.ph_len = pup->ud_len;
	
	csum = pup->ud_cksum;		/* check the checksum */
	if (csum != 0) {		/* goddam checksums may be unused! */
		pup->ud_cksum = cksum (&ph, sizeof (ph) >> 1, 0);
		if (udplen > BIGCKSUM)	/* see if fast cksum routine better */
			pup->ud_cksum = ~fcksum (pup, (udplen + 1) >> 1, 0);
		else
			pup->ud_cksum = ~cksum (pup, (udplen + 1) >> 1, 0);
		if (pup->ud_cksum != csum) 
			return (0);
	}
	
#ifndef	BIGINDIAN
	udpswab (pup);			/* swap bytes if needed */
#endif
	
	return (datalen);
}