// linux 系統編譯通過
// gcc filename.c -o filename
// ./filename

#include <stdlib.h>
#include <stdio.h>

unsigned char ip_hdr_8[] = 
{
	0x45,0x00,
	0x00,0x3c,
	0x00,0x00,
	0x00,0x00,
	0x40,0x01,
	0x00,0x00,	// checksum
	0xc0,0xa8,
	0x01,0x25,
	0xda,0x3c,
	0x06,0x82
};

unsigned short ip_hdr_16[] = 
{
	0x4500,
	0x003c,
	0x0000,
	0x0000,
	0x4001,
	0x0000,		// checksum
	0xc0a8,
	0x0125,
	0xda3c,
	0x0682
};

unsigned int checksum_8(unsigned int cksum, void *pBuffer, unsigned int size)
{
    char num = 0;
    unsigned char *p = (unsigned char *)pBuffer;
 
    if ((NULL == pBuffer) || (0 == size))
    {
        return cksum;
    }
   
    while (size > 1)
    {
        cksum += ((unsigned short)p[num] << 8 & 0xff00) | (unsigned short)p[num + 1] & 0x00FF;
        size  -= 2;
        num   += 2;
    }
   
	if (size > 0)
    {
        cksum += ((unsigned short)p[num] << 8) & 0xFFFF;
        num += 1;
    }
 
    while (cksum >> 16)
    {
        cksum = (cksum & 0xFFFF) + (cksum >> 16);
    }
   
    return ~cksum;
}


unsigned short checksum_16(unsigned short *addr,int len)
{
	unsigned short chksum;
	unsigned int sum = 0;

	while(len > 1)
	{
		sum += *addr++;
		len -= 2;
	}
	
	if(len == 1)
		sum += *(unsigned char*)addr;
		
	sum = (sum>>16) + (sum & 0xffff);
	sum += (sum>>16);
	chksum = ~sum;
	return (chksum);
}

void main()
{
	unsigned short chksum;

	printf("--------------------------------------------\n");
	
	chksum = checksum_8(0,ip_hdr_8,sizeof(ip_hdr_8));
	printf("checksum_8 = 0x%04x \n",chksum);
	ip_hdr_8[10] = chksum >> 8;
	ip_hdr_8[11] = chksum & 0xff;
	
	chksum = checksum_8(0,ip_hdr_8,20);
	if(chksum)
		printf("Checksum_8 is incorrect! \n");
	else
		printf("Checksum_8 is correct! \n");

	printf("--------------------------------------------\n");
	
	chksum = checksum_16(ip_hdr_16,sizeof(ip_hdr_16));
	printf("checksum_16 = 0x%04x \n",chksum);
	ip_hdr_16[5] = chksum;
	
	chksum = checksum_16(ip_hdr_16,20);
	if(chksum)
		printf("Checksum 16 is incorrect! \n");
	else
		printf("Checksum 16 is correct! \n");
	
	printf("--------------------------------------------\n");
}