Internet checksum Algorithm Implementation

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. /*
3. Pbuffer is the start address of the datagram to be verified.
5. Nsize specifies the length of the verification content, in bytes
7. */
9. Unsigned short checksum_calculating (unsigned short * pbuffer, int nsize)
10. {
11. Unsigned long dwcksum = 0; // 32-bit sum
13. Unsigned char * P = (unsigned char *) & dwcksum;
14. // Accumulate in two bytes
15. While (nsize> 1)
16. {
17. Dwcksum + = * pbuffer ++;
18. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
19. Nsize-= sizeof (unsigned short );
20. }
21. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
22. // If the total number of bytes is an odd number, add the last byte.
23. If (nsize)
24. {
25. Dwcksum + = * (unsigned char *) pbuffer;
26. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
28. }
30. // Accumulate the carry in the 32-bit integer high to the 16-bit lower
31. While (dwcksum> 16 ){
32. Dwcksum = (dwcksum & 0 xFFFF) + (dwcksum> 16 );
33. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
34. }
36. // Obtain the inverse checksum.
37. Dwcksum = ~ Dwcksum;
38. Printf ("/T %. 2x %. 2x %. 2x %. 2x/N ", * P, * (p + 1), * (p + 2), * (p + 3 ));
40. // Returns the 16-bit checksum.
41. Return (unsigned short) (dwcksum );
42. }