32 Bit CRC calculation not matching with online generator - c

Question

Can somebody help me in this calculation of 32 bit CRC.
This is the peice of code I used for 32 bit CRC calculation.
static unsigned int crc32_table[256];
void make_crc_table()
{
int j;
unsigned int crc,byte, mask;
/* Set up the table, if necessary. */
if (crc32_table[1] == 0)
{
for (byte = 0; byte <= 255; byte++)
{
crc = byte;
for (j = 7; j >= 0; j--) // Do eight times
{
mask = -(crc & 1);
crc = (crc >> 1) ^ (0xEDB88320 & mask);
}
crc32_table[byte] = crc;
}
}
for (j=0;j<10;j++)
printf("crc32_table[%d] = %x\n",j,crc32_table[j]);
}
unsigned int crc32cx(unsigned int crc,unsigned char *message,int len)
{
unsigned int word;
do
{
if((word = *(unsigned int *)message) & 0xFF)
{
crc = crc ^ word;
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
message = message + 4;
len--;
}
}while(len == 0);
return ~crc;
}
main()
{
unsigned int crc = 0xFFFFFFFF;
unsigned char buff[100] = ABCDEFGH;
int len; // lenght in bytes
len = (((strlen(buff)%8)==0) ? (strlen(buff)/8) : ((strlen(buff)/8)+1));
printf("lenght in bytes %d\n",len);
make_crc_table();
printf("crc = %x\n",crc32cx(crc,buff,len));
}
Can somebody help me why this is not matching with online 32 bit CRC calculator. Link given below
http://www.tahapaksu.com/crc/
For input buff=12345678, my CRC is getting matched with the online one.
For other values like buff = ABCD1234, the output is not matching.
Thanks.

Answer 1

The problem here is the way the code is written; let me explain:
unsigned int crc32cx(unsigned int crc,unsigned char *message,int len)
{
unsigned int word;
do
{
if((word = *(unsigned int *)message) & 0xFF)
{
crc = crc ^ word;
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
message = message + 4;
len--;
}
}while(len == 0);
return ~crc;
}
What this function does is to read 4 chars at a time and them compute the CRC (XOR operation); Wikipedia explains the math behind it.
But you do this operation len times
unsigned char buff[100] = ABCDEFGH;
int len; // lenght in bytes
printf("crc = %x\n",crc32cx(crc,buff,4));
So in your case, you will read 4x4 bytes; And your buffer will contain:
buff = ['A' 'B' 'C' 'D' 'E' 'F' 'G' 'H' '\n' 'trash' 'trash'.... ]
You have 8 bytes of information, followed by a '\n' since you are assigning the buffer a string, than trash, since the buffer is allocated on the stack. And you are reading 16 bytes.
I'm sure you can spot the problem by now, but just in case, I think that crc32cx(crc,buff,2) should solve your issue.

Answer 2

Your CRC code is very nonstandard. When doing a table method you are suppose to input the data byte by byte, not 4 byte chunk by chunk which is definitely causing some input and logic issues. The biggest one being this line if(word = *(unsigned int *)message) & 0xFF) which is completely unnecessary and will ignore valid incoming data in some scenarios.
A nice, simple, and clean crc32 C implementation can be seen here. After looking at it and yours and making a few tweaks it worked.
In your function you would change the loop and your variable to:
unsigned char word;
do
{
word = *message;
crc = crc ^ word;
crc = (crc >> 8) ^ crc32_table[crc & 0xFF];
message++;
len--;
}while(len > 0);
And now in your main you can find the length of your input data just using len = strlen(buff).