Shulou(Shulou.com)06/01 Report--

Checksum check algorithm for SQL SERVER data pages

In versions above SQL SERVER2005, checksum is enabled by default on the data page, identified as m_flagBits & 0x200 = = True, and its value m_tornBits is located in the header 0x3CMagi 4 bytes.

The algorithm is summarized as follows:

Read 8KB into BUF to make the 4-byte value of the BUF header CHECKSUM clear 0uint32 checksum = 0 / / initial checksumfor i in range (0Magne15): / / initial checksum overall of each sector = 0; for ii in range (0127): / / A cumulative difference or overall = overall ^ BUF [I] [ii] to each 4 bytes of the current sector / / A pair of checksum of each sector is shifted by shifting 15muri bits to the left, and / / the 15muri bits moved to the left are added to the lowest bit. Checksum = checksum ^ rol (overall, 15-I); return checksum; / / Gets checksum

C source code is as follows:

/ / * CODE***//#include # include # define seed 15 / / Initial seed (for first sector) # define CHAR_BIT 8 / / * PROTOTYPES***//unsigned int page_checksum (int page_id, unsigned int * ondisk); unsigned int rol (unsigned int value, unsigned int rotation); int main (int argc, char * argv []) {unsigned int computed_checksum; / / Var to retrieve calculated checksum unsigned int ondisk_checksum / / Var to retrieve checksum ondisk computed_checksum = page_checksum (152, & ondisk_checksum); / / page_checksum call to retrieve stored and calculated checksum for page 152 / / * PRINTS***// printf ("Calculated checksum: 0xx\ n", computed_checksum); printf ("On disk checksum: 0xx\ n", ondisk_checksum);} unsigned int page_checksum (int page_id, unsigned int * ondisk) {FILE * fileptr; unsigned int i Unsigned int j; unsigned int checksum; unsigned int overall; unsigned int * pagebuf [16]; / / A pointer to describe 2d array [sector] [element] fileptr = fopen ("C:\\ Users\\ andre\\ Desktop\\ teste.mdf"); / / Open dummy data file for binary read fseek (fileptr, page_id * 8192, SEEK_SET); / / Calculate page address on data file and points to it fread (pagebuf, 4, 2048, fileptr) / / Read page buffer fclose (fileptr); checksum = 0; overall = 0; * ondisk = pagebuf [0] [15]; / / This means that torn bits is stored on first sector in 15th element, Internals researches understand this pagebuf [0] [15] = 0x000000000; / / Fill checksum field with zeroes (this field will be discarded in algorithm) for (I = 0; I

< 16; i++) //Loop through sectors { overall = 0; //Reset overall sum for sectors for (j = 0; j < 128; j++) //Loop through elements in sector i { overall = overall ^ (unsigned int)pagebuf[i][j]; //XOR operation between sector i elements } checksum = checksum ^ rol(overall, seed - i); //Current checksum is overall for sector i circular shifted by seed (15 - i) } return checksum; //Gets checksum}unsigned int rol(unsigned int value, unsigned int rotation){ return (value) >

(sizeof (int) * CHAR_BIT-rotation) & (1

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.