C :: How To Create Packet Data

How can header info 'foo' be combined with data in buffer 'buff' to pass to sendto function

Code:

/* Send a single RTP packet, converting the RTP header to network byte order. */
int sendrtp(int fd, struct sockaddr_in ........ockAddr, struct rtpheader *foo, void *data, int len)
{
char *buff = alloca(len+sizeof(struct rtpheader));
foo->b.sequence = htons(foo->b.sequence);
foo->timestamp = htonl(foo->timestamp);
foo->ssrc = htonl(foo->ssrc);
}

[code]....