sendto() returns "Invalid argument" when used to send UDP on raw socket

I found a reference (http://www.tenouk.com/Module43a.html) for using raw sockets to send a simple UDP packet in Linux, but I cannot get it to work on macOS Sierra10.12.1. I've tried a number of different variations, but sendto() always returns Invalid argument.

I'd be grateful if you could point out my mistake.

Thanks!

#include <iostream>
#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/udp.h>
#include <arpa/inet.h>

#define PCKT_LEN 8192

struct ipheader {
    unsigned char      iph_ihl:5, iph_ver:4;
    unsigned char      iph_tos;
    unsigned short int iph_len;
    unsigned short int iph_ident;
    unsigned char      iph_flag;
    unsigned short int iph_offset;
    unsigned char      iph_ttl;
    unsigned char      iph_protocol;
    unsigned short int iph_chksum;
    unsigned int      iph_sourceip;
    unsigned int      iph_destip;
};

struct udpheader {
    unsigned short int udph_srcport;
    unsigned short int udph_destport;
    unsigned short int udph_len;
    unsigned short int udph_chksum;
};

unsigned short csum(unsigned short *buf, int nwords)
{
    unsigned long sum;
    for(sum=0; nwords>0; nwords--)
        sum += *buf++;
    sum = (sum >> 16) + (sum &0xffff);
    sum += (sum >> 16);

    return (unsigned short)(~sum);
}

int main(int argc, char *argv[])
{
    int sd;
    char buffer[PCKT_LEN], *data;  

    struct ipheader *ip = (struct ipheader *) buffer;
    struct udpheader *udp = (struct udpheader *) (buffer + sizeof(struct ipheader));
    data = ((char *)udp + sizeof(struct udpheader));
    struct sockaddr_in sin, din;
    int one = 1;
    const int *val = &one;

    memset(buffer, 0, PCKT_LEN);

    printf("*** start Debug ***\n");
    printf("char: %lu\n", sizeof(char));
    printf("short: %lu\n", sizeof(short));
    printf("unsigned short: %lu\n", sizeof(unsigned short));
    printf("int: %lu\n", sizeof(int));
    printf("unsigned int: %lu\n", sizeof(unsigned int));
    printf("long: %lu\n", sizeof(long));
    printf("unsigned long: %lu\n", sizeof(unsigned long));
    printf("*** end Debug ***\n");

    /*
    if(argc != 5)
    {
        printf("- Invalid parameters!!!\n");
        printf("- Usage %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);
        exit(-1);
    }
    int srcPort = atoi(argv[2]);
    int destPort = atoi(argv[4]);
    char *str_srcIP = argv[1];
    char *str_destIP = argv[3];
    */

    int srcPort = 23;
    int destPort = 23;
    char str_srcIP[] = "192.168.1.72";
    char str_destIP[] = "192.168.1.73";

    sd = socket(PF_INET, SOCK_RAW, IPPROTO_UDP);
    if(sd < 0)
    {
        perror("socket() error");
        exit(-1);
    }
    else
        printf("socket() - Using SOCK_RAW socket and UDP protocol is OK.\n");

    sin.sin_len = sizeof(sin);
    din.sin_len = sizeof(din);
    sin.sin_family = AF_INET;
    din.sin_family = AF_INET;

    sin.sin_port = htons(srcPort);
    din.sin_port = htons(destPort);

    sin.sin_addr.s_addr = inet_addr(str_srcIP);
    din.sin_addr.s_addr = inet_addr(str_destIP);

    strcpy(data, "A");

    unsigned long payloadSize = sizeof(struct ipheader) + sizeof(struct udpheader) + strlen(data);

    ip->iph_ihl = 5;
    ip->iph_ver = 4;
    ip->iph_tos = 0;
    ip->iph_len = payloadSize;
    ip->iph_ident = htons(54321);
    ip->iph_ttl = 64;
    ip->iph_protocol = IPPROTO_UDP;
    ip->iph_sourceip = inet_addr(str_srcIP);
    ip->iph_destip = inet_addr(str_destIP);

    udp->udph_srcport = htons(srcPort);
    udp->udph_destport = htons(destPort);
    udp->udph_len = htons(sizeof(struct udpheader));

    ip->iph_chksum = csum((unsigned short *)buffer, sizeof(struct ipheader) + sizeof(struct udpheader));

    if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
    {
        perror("setsockopt() error");
        exit(-1);
    }
    else
        printf("setsockopt() is OK.\n");

    printf("Trying...\n");
    printf("Using raw socket and UDP protocol\n");
    printf("Using Source IP: %s port: %u, Target IP: %s port: %u.\n", str_srcIP, srcPort, str_destIP, destPort);

    std::cout<<"*** start Debug ***"<<std::endl;
    std::cout<<"sd: "<<sd<<std::endl;
    std::cout<<"Buffer: "<<buffer<<std::endl;
    std::cout<<"iph_len: "<<ip->iph_len<<std::endl;
    std::cout<<"sin: "<<(struct sockaddr *)&sin<<std::endl;
    std::cout<<"sizeof(sin): "<<sizeof(sin)<<std::endl;
    std::cout<<"*** end Debug ***"<<std::endl;

    int count;
    for(count = 1; count <=20; count++)
    {
        if(sendto(sd, buffer, ip->iph_len, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)
        {
            perror("sendto() error");
            exit(-1);
        }
        else
        {
            printf("Count #%u - sendto() is OK.\n", count);
            sleep(2);
        }
    }
    close(sd);
    return 0;
}