Bug 30634

When kevent returns a read event for a UDP socket, the data field (AKA the number of bytes to be read) is 16 bytes too long.

NOTE:  the problem exists in 4.3 release as well.

How-To-Repeat: 1)  Compile up the following discard server 
2)  Run nc -u 127.0.0.1 6060
3)  Send data to netcat and watch the server's output

It should be consistently off by sixteen bytes.  NOTE:  this may be a documentation bug and "the number of bytes in the socket buffer" needs to be explained further.   

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/event.h>

/* loopback discardserver on port 6060 */
int main(void)
{
    int queue,sock;
    struct sockaddr_in saddr;
    struct kevent evt;

    memset(&saddr,0,sizeof(struct sockaddr_in));
    saddr.sin_family = AF_INET;
    saddr.sin_port = htons(6060);
    saddr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

    memset(&evt,0,sizeof(struct kevent));
    evt.filter = EVFILT_READ;
    evt.flags =  EV_ADD;

    if(((queue = kqueue()) == -1) ||
       ((evt.ident = (sock = socket(AF_INET,SOCK_DGRAM,0))) == -1) ||
       (bind(sock,(struct sockaddr *)&saddr,sizeof saddr)) ||
       (kevent(queue,&evt,1,NULL,0,NULL)))
        return(fprintf(stderr,"bad setup\n"),EXIT_FAILURE);

    while(1){
        char *buf;

        if((kevent(queue,NULL,0,&evt,1,NULL) == -1) ||
           (!evt.data))
            break;

        if((buf = malloc(evt.data)) == NULL) continue;

        recv(sock,buf,evt.data,0);
        printf("size to read(%d) : data : %s\n",evt.data,buf);

        free(buf);
    }

    return(EXIT_SUCCESS);
}