Calculating Checksum Java

flyz

I'm sending a byte array to a C server and they are calculating the checksum using the following algorithm :

uint32 CheckSum32(uint32* p_buf, uint nbytes)
{
uint i;
uint32 sum = 0;

for(i = 0; i < nbytes/4; i++){
sum += p_buf;
}

return sum;
}

I need to calculate the same checksum on my Java side

What I'm doing at the moment is :

long checksum = 0;
for(int c = 0; c < byte_buffer.length/4; c+=4)
{
checksum += convertHexArrayToLong(byte_buffer, 4, c);
}

The function convertHexArrayToLong takes 4 bytes ( 0x23 , 0x45, 0x51, 0x94) and converts it to a long in the format 0x23455194.

I've tried using an int value as well but no joy.

I'm thinking it's to do with the different range if values for a uint32 in C and an int or long in Java.

Can someone look at the C code above and suggest what I could do in Java to calculate the same checksum please?

Zab

Well, I have a few observations anyway.

You are correct about the range of a uint32 and a java int being an issue. However, you can get around this in java by using a long ( as you are ) and after the += calculation do a

checksum &= 0xFFFFFFFF;

This will snip off the bits that are over what a uint32 can hold.

I'm unsure what you are doing with the nbytes in the C code. If this is actually the number of bytes ( with four bytes in each uint32 ) then the java code is only iterating through a quarter of what the C code is iterating.

Also, the convertHexArrayToLong is a little confusing, as I don't see a hex array anywhere... I see four bytes that you want to turn into a long. You should also keep the upper bound in mind ( what happens if the array is not divisable by four ).

Zab.

flyz

Originally posted by Zab
Well, I have a few observations anyway.

You are correct about the range of a uint32 and a java int being an issue. However, you can get around this in java by using a long ( as you are ) and after the += calculation do a

checksum &= 0xFFFFFFFF;

This will snip off the bits that are over what a uint32 can hold.

thanks I'll give that a try

I'm unsure what you are doing with the nbytes in the C code. If this is actually the number of bytes ( with four bytes in each uint32 ) then the java code is only iterating through a quarter of what the C code is iterating.

To get the checksum you take 4 bytes at a time and add them together. Each 'element' in the uint32* p_buf is 4 bytes long.

Also, the convertHexArrayToLong is a little confusing, as I don't see a hex array anywhere... I see four bytes that you want to turn into a long. You should also keep the upper bound in mind ( what happens if the array is not divisable by four ).

Zab.

sorry the parameters for the convertHexArrayToLong are :

long convertHexArrayToLong( byte[] byte_array, int numBytes, int offset);

I also pad the array out to be 4 byte aligned before I calculate the checksum.

Zab

Fair enough. However, you are still only iterating a quarter of the array in the java version. And there is no "hex array", just an array of bytes. I'm sure the method works, it's just the name that I have an issue with.

flyz

Originally posted by Zab
Fair enough. However, you are still only iterating a quarter of the array in the java version. And there is no "hex array", just an array of bytes. I'm sure the method works, it's just the name that I have an issue with.

Yes you were right, I think I did so many things to try and make it work that I forgot to change things back

this is what I've finished with anyway.
you have to specify mask as : 0xFFFFFFFL otherwise it won't work.

long checksum = 0;
long temp_checksum = 0;
for(int c = 0; c < temp_array.length; c+=4)
{
  temp_checksum = SesPages.convertHexArrayToLon(temp_array, 4, c);
     
 checksum = (checksum + temp_checksum)&0xFFFFFFFFL;
}

Zab

I'm presuming it works? I remembered that it would need the L after I posted, but forgot to mention it in my second post, sorry.

CH

i found this tutorial usefull a few months ago: http://www.acte.no/teknisk/html/crcguide.htm

int DATA_SIZE = 8;
int CHK_SEED = 127;

long l; 
int chkSum; 

while( run ) 
{ 
	l = 0;				// reset 
	chkSum = 0; 

	readData();			// blocking call: read data frame
	buffer.rewind();		// position = 0 

	// calc chksum for frame
	while( buffer.position() < ( DATA_SIZE - 1 ) ) 
	{ 
		l = ( l << 8 ) + ( buffer.get() & 0xFF ); 
	} 
	chkSum = ( int )( l % CHK_SEED );

	// compare chkSum with last byte in frame
	if( chkSum == buffer.get() )	// postition = 7
	{ 
		// chksums match
	} 
	else
	{
		// chksums dont match
	}
}