Example.

After setting socket option IPV6_FLOWINFO flowlabel and DS field are received as ancillary data object of type IPV6_FLOWINFO and level SOL_IPV6. In the cases when it is convenient to use recvfrom(2), it is possible to replace library variant with your own one, sort of:

#include <sys/socket.h>
#include <netinet/in6.h>

size_t recvfrom(int fd, char *buf, size_t len, int flags,
                struct sockaddr *addr, int *addrlen)
{
  size_t cc;
  char cbuf[128];
  struct cmsghdr *c;
  struct iovec iov = { buf, len };
  struct msghdr msg = { addr, *addrlen,
                        &iov,  1,
                        cbuf, sizeof(cbuf),
                        0 };

  cc = recvmsg(fd, &msg, flags);
  if (cc < 0)
    return cc;
  ((struct sockaddr_in6*)addr)->sin6_flowinfo = 0;
  *addrlen = msg.msg_namelen;
  for (c=CMSG_FIRSTHDR(&msg); c; c = CMSG_NEXTHDR(&msg, c)) {
    if (c->cmsg_level != SOL_IPV6 ||
      c->cmsg_type != IPV6_FLOWINFO)
        continue;
    ((struct sockaddr_in6*)addr)->sin6_flowinfo = *(__u32*)CMSG_DATA(c);
  }
  return cc;
}