Example recvmsg() code for Linux and Unix

recvmsg.c

void
recv_cb(evutil_socket_t fd, short event, void *arg)
{
    struct msghdr msg;
    struct iovec iov[1];
    ssize_t len;
    int flags = 0;
    struct cmsghdr *cmptr;
    sockaddr_union_t sau;
    struct unp_in_pktinfo {
        union addr_union {
            struct in_addr  ipi_addr;
            struct in6_addr ipi_addr6;
        } au;
        int ipi_ifindex;
    } pkt;
    union {
        struct cmsghdr cm;
        u_int8_t pktinfo_sizer[sizeof(struct cmsghdr) + 1024];
    } control_un;
    uint16_t port = 0;
    char dst[INET6_ADDRSTRLEN], src[INET6_ADDRSTRLEN];
    u_int8_t *buf = calloc(1, 4096);

    msg.msg_control = &control_un;
    msg.msg_controllen = sizeof(control_un);
    msg.msg_flags = 0;

    msg.msg_name = &sau.sa;
    msg.msg_namelen = sizeof(sau);
    iov[0].iov_base = buf;
    iov[0].iov_len = MAX_PKT_SIZE - 1;
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    len = recvmsg(fd, &msg, flags);
    if (len < 0) {
        perror("recvmsg");
        goto done;
    } else if (len == 0) {
        fprintf(stderr, "recvmsg len 0, Connection closed");
        goto done;
    }

    bzero(&pkt, sizeof(struct unp_in_pktinfo));
    flags = msg.msg_flags;
    if (msg.msg_controllen < sizeof(struct cmsghdr)) {
        fprintf(stderr, "received truncated msg len: %d", msg.msg_controllen);
        goto done;
    }

    if (flags & MSG_TRUNC) {
        fprintf(stderr, "received truncated datagram: 0x%0x", flags);
        goto done;
    }

    if (flags & MSG_CTRUNC) {
        fprintf(stderr, "received truncated control info: 0x%0x", flags);
    }

    for (cmptr = CMSG_FIRSTHDR(&msg); cmptr != NULL;
         cmptr = CMSG_NXTHDR(&msg, cmptr)) {
#ifdef SOL_IP
    if ((cmptr->cmsg_level == SOL_IP) &&
	(cmptr->cmsg_type == IP_PKTINFO)) {
	    struct in_pktinfo *i = (struct in_pktinfo *) CMSG_DATA(cmptr);
            memcpy(&pkt.au.ipi_addr, &i->ipi_addr, sizeof(struct in_addr));
            pkt.ipi_ifindex = i->ipi_ifindex;
	    break;
	}
#endif
#ifdef  IP_RECVDSTADDR
        if (cmptr->cmsg_level == IPPROTO_IP &&
            cmptr->cmsg_type == IP_RECVDSTADDR) {

            memcpy(&pkt.au.ipi_addr, CMSG_DATA(cmptr), sizeof(struct in_addr));
            continue;
        }
#endif

#ifdef  IP_RECVIF
        if (cmptr->cmsg_level == IPPROTO_IP &&
            cmptr->cmsg_type == IP_RECVIF) {
            struct sockaddr_dl *sdl;

            sdl = (struct sockaddr_dl *)CMSG_DATA(cmptr);
            pkt.ipi_ifindex = sdl->sdl_index;
            continue;
        }
#endif

#ifdef	IPV6_PKTINFO
        if (cmptr->cmsg_level == IPPROTO_IPV6 &&
            cmptr->cmsg_type == IPV6_PKTINFO) {
            struct in6_pktinfo *pkt_info;

            pkt_info = (struct in6_pktinfo *)CMSG_DATA(cmptr);
            memcpy(&pkt.au.ipi_addr6, &pkt_info->ipi6_addr, sizeof(struct in6_addr));
            pkt.ipi_ifindex = pkt_info->ipi6_ifindex;
            continue;
        }
#endif

    }
done:
}