CRC for a file

[pradeepkrao]

Hi,

Need to know how to do a CRC on a file ?.

I need the code in C++ or C or in VB.

Any help is appreciated..

Thanks,
Pradeep

[jim mcnamara]

First off -
There is no one CRC algorithm - they are 16 bit or 32 bit.
Then a polynomial is applied (mask). There are a lot of polynomials used

As long as you use the same CRC32 and the same polynomial, results for identical data streams match. If not, all bets are off.

Attached is a CRC written in VB

[jim mcnamara]

A more robust C implementation

/* crc-32 for files  using autodin II, NIC polynomial 04/16/2002 4:33AM jmc*/

#include <stdio.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <string.h>
#define U_LONG unsigned int
#define U_CHAR unsigned char
#define CRC32_POLY 0x04c11db7     /* AUTODIN II, Ethernet, & FDDI */
static U_LONG crc32_table[256];
static struct stat a;

void init_crc32(void);
void test(FILE *);
U_LONG crc32(U_CHAR*,int);

int main(int argc,char *argv[]){
       FILE *in;
       char *buf;
       if (stat(argv[1],&a)) {
       	   printf("Cannot stat %s\n",argv[1]);
       	   perror("Usage: crc32 <filename>  \nError:");
       	   exit(EXIT_FAILURE);
       	}
       test(in=fopen(argv[1],"rb") );
       buf=(char*)malloc(a.st_size);
       fread(buf,1,a.st_size,in);
       fclose(in);
       init_crc32();   /* build table */
       printf("%X\n", crc32((U_CHAR*)buf,a.st_size));
       free(buf);
       return 0;	
}


U_LONG crc32(U_CHAR *buf, int len)
{
        U_CHAR *p;
        U_LONG  crc;
        crc = 0xffffffff;       /* preload shift register, per CRC-32 spec */
        for (p = buf; len > 0; ++p, --len)
                crc = (crc << 8) ^ crc32_table[(crc >> 24) ^ *p];
        return ~crc;            /* transmit complement, per CRC-32 spec */
}

/*
 * Build auxiliary table for parallel byte-at-a-time CRC-32.
 */


void init_crc32()
{
        int i, j;
        U_LONG c;

        for (i = 0; i < 256; ++i) {
                for (c = i << 24, j = 8; j > 0; --j)
                        c = c & 0x80000000 ? (c << 1) ^ CRC32_POLY : (c << 1);
                crc32_table[i] = c;
        }
}
/*  validate file open, exit on bad file */
void test(FILE *tst){
	if(tst==NULL) {
		perror("Error opening input file:");
		exit(EXIT_FAILURE);
	}
}

[pradeepkrao]

Hae,

Thanks a Lot..

The C code always return's 0 on any file.. But the VB one works fine.. Could you please check the C version ..


Thanks,
Pradeep

[pradeepkrao]

Woww.... its working fine now...

I forgot to copy stat.. and hence.....

Hae Thanks a Lot...

Pradeep

[jim mcnamara]

Returning zero means it succeeded, in unix terms and in standard ANSI C terms as well.

Plus, that code has been in production for a long time - I don't think there's too much wrong with it.

All the code does is print the CRC.

[pradeepkrao]

Thanks Jim

[amitabh]

You can also use MD5 Hash for checking the authenticity of the file.