# Internet checksum Algorithm Implementation

Source: Internet
Author: User

Enhanced Internet checksum algorithm 8.10.20

For the principles, see:

Internet checksum algorithm: http://blog.csdn.net/microtong/archive/2008/10/20/3112139.aspx

Features of Internet checksum algorithm: http://blog.csdn.net/microtong/archive/2008/10/20/3112157.aspx

1. /*
2. Pbuffer is the start address of the datagram to be verified.
3. Nsize specifies the length of the verification content, in bytes
4. */
5. Unsigned short checksum_calculating (unsigned short * pbuffer, int nsize)
6. {
7. Unsigned long dwcksum = 0; // 32-bit sum
8. Unsigned char * P = (unsigned char *) & dwcksum;
9. // Accumulate in two bytes
10. While (nsize> 1)
11. {
12. Dwcksum + = * pbuffer ++;
13. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
14. Nsize-= sizeof (unsigned short );
15. }
16. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
17. // If the total number of bytes is an odd number, add the last byte.
18. If (nsize)
19. {
20. Dwcksum + = * (unsigned char *) pbuffer;
21. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
22. }
23. // Accumulate the carry in the 32-bit integer high to the 16-bit lower
24. While (dwcksum> 16 ){
25. Dwcksum = (dwcksum & 0 xFFFF) + (dwcksum> 16 );
26. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
27. }
28. // Obtain the inverse checksum.
29. Dwcksum = ~ Dwcksum;
30. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
31. // Returns the 16-bit checksum.
32. Return (unsigned short) (dwcksum );
33. }
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