Converting from 2's complement to decimal integer

**clancyPC** · 05-05-2011

Hi,
I am converting data from an asn1 stream but my problem is with converting integer values. Asn1 works on a Tag, Length, Value (TLV) notation, so an integer can be stored in a variable length field. When that length is 1, 2 or 4 the conversion to a decimal value is relatively straight forward I am just doing this:

Code:

    ber_tlv_tag_t TagSize;
    ber_tlv_tag_t TagValue;
    signed short ssTempInt = 0;
    signed int   siTempInt = 0;


           switch (TagSize)
           {
               case 1:  TagValue =  *(const uint8_t *)&InputBuffer[CharsUsed];
                        ThisEvent->DataVolumeGprsUplink = (int) TagValue;
                        break;
               case 2:  ssTempInt =  *(const uint16_t *)&InputBuffer[CharsUsed];
                        ThisEvent->DataVolumeGprsUplink = (int) ssTempInt;
                        break;
               case 4:  siTempInt =  *(const uint32_t *)&InputBuffer[CharsUsed];
                        ThisEvent->DataVolumeGprsUplink = (int) siTempInt;
                        break;
               default: TRACE(("Tagsize is not easily converted\0"));
            }

Basically casting it to the appropriately sized variable. However when the variable is not of a standard size eg 3 bytes I am not entirely sure what to do? Should I do a 2's complement conversion to a decimal integer? Thoughts, ideas, suggestions appreciated?
Thanks

**MK27** · 05-05-2011

Since these are already in a char array, why not use:

Code:

union num {
	int val;
	char data[4];
};

Now pretend that "input" is your data, 4 24-bit numbers in row:

Code:

int main(void) {
	char input[12] = "hello world";
	int i, j;
	union num eg;

	for (i = 0; i < 4; i++) {
		eg.val = 0;    // zero out the union
		for (j = 0; j < 3; j++) eg.data[j] = input[j+i*3];
		printf("%d\n",eg.val);
	}

	return 0;
}

**grumpy** · 05-05-2011

It call comes down to defining what a set of TagSize bytes represents for each value of TagSize.

I assume that

1) InputBuffer is an array of unsigned char.
2) ThisEvent->DataVolumeGprsUplink is of a type that can hold any integral value represented by TagSize unsigned chars.

Then I would probably do something like

Code:

ThisEvent->DataVolumeGprsUplink = 0;
for (i = 0; i < TagSize; ++i)
{
      ThisEvent->DataVolumeGprsUplink *= UCHAR_MAX;   /*  UCHAR_MAX is maximum value of unsigned char from <limits.h>
      ThisEvent->DataVolumeGprsUplink += InputBuffer[CharsUsed + i];
}

This assumes the byte order is most significant first. If first byte is least significant, reverse the order through the loop. If you have some other byte order corresponding to values, you will need to accommodate that in some other way.

Incidentally, your approach can fail if int a type that is no bigger than 32 bits (which is common in practice) or short is less than 32 bits (which is very common in practice) . That is one reason why I would avoid using anything like your switch statement.

**clancyPC** · 05-05-2011

Grumpy,
I see what you are doing, its effectively in decimal terms, reading the first three digits and then multiplying by 1000 before getting the next three digits. There is probably a more technically correct way of putting that, but thats how I can visualise it in my head in a way that makes sense. The only thing I found was that on my box UCHAR_MAX is set to 255, which is the wrong value it needs to be 256 to work properly.
Thanks for your help

**mike65535** · 05-05-2011

UCHAR_MAX is set to 255, which is the wrong value it needs to be 256 to work properly

Well since a char is 8 bits, the largest value it can hold is 255. Not sure what you think you can do about that.

**grumpy** · 05-05-2011

Clancy's point - correctly - was that the value I needed to multiply by was UCHAR_MAX + 1 (the sample code I gave was off by 1).

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