not able to shift hex data in a unsigned long

Question

i am trying to convert IEEE 754 Floating Point Representation to its Decimal Equivalent so i have an example data [7E FF 01 46 4B CD CC CC CC CC CC 10 40 1B 7E] which is in hex.

char strResponseData[STATUS_BUFFERSIZE]={0};

unsigned long strData = (((strResponseData[12] & 0xFF)<< 512 ) |((strResponseData[11] & 0xFF) << 256) |((strResponseData[10] & 0xFF)<< 128 ) |((strResponseData[9]  & 0xFF)<< 64) |((strResponseData[8] & 0xFF)<< 32 ) |((strResponseData[7]& 0xFF) << 16) |((strResponseData[6] & 0xFF )<< 8) |(strResponseData[5] & 0xFF));

value = IEEEHexToDec(strData,1);

then i am passing this value to this function

IEEEHexToDec(unsigned long number, int isDoublePrecision)
{
    int mantissaShift = isDoublePrecision ? 52 : 23;
    unsigned long exponentMask = isDoublePrecision ? 0x7FF0000000000000 : 0x7f800000;
    int bias = isDoublePrecision ? 1023 : 127;
    int signShift = isDoublePrecision ? 63 : 31;

    int sign = (number >> signShift) & 0x01;
    int exponent = ((number & exponentMask) >> mantissaShift) - bias;

    int power = -1;
    double total = 0.0;
    for ( int i = 0; i < mantissaShift; i++ )
    {
        int calc = (number >> (mantissaShift-i-1)) & 0x01;
        total += calc * pow(2.0, power);
        power--;
    }
    double value = (sign ? -1 : 1) * pow(2.0, exponent) * (total + 1.0);

    return value;
}

but in return am getting value 0, also when am trying to print strData it is giving me only CCCCCD. i am using eclipse ide. please i need some suggestion

it looks like C, but you should definitely tag with a programming language! — B. Go, Dec 09 '19 at 16:16

Chris Stratton · Answer 1 · 2019-12-09T17:13:49.680

((strResponseData[12] & 0xFF)<< 512 )

First, the << operator takes a number of bits to shift, you seem to be confusing it with multiplication by the resulting power of two - while it has the same effect, you need to supply the exponent. Given that you have no typical data types of 512 bit width, it's fairly certain that this should actually be.

((strResponseData[12] & 0xFF)<< 9 )

Next, it's necessary for the value to be shifted to be of a sufficient type to hold the result before you do the shift. A char is obviously not sufficient, so you need to explicitly cast the value to a sufficient type to hold the result before you perform the shift.

Additionally keep in mind that depending on your platform an unsigned long may be either a 32 bit or 64 bit type, so if you were doing an operation with a bit shift where the result would not fit in 32 bits, you may want to use an unsigned long long or better yet make things unambiguous, for example with #include <stdint.h> and type such as uint32_t or uint64_t. Given that your question is tagged "embedded" this is especially important to keep in mind as you might be targeting a 32 (or even 8) bit processor, but sometimes building algorithms to test on the development machine instead.

Further, a char can be either a signed or an unsigned type. Before shifting, you should make that explicit. Given that you are combining multiple pieces of something, it is almost certain that at least most of these should be treated as unsigned.

So probably you want something like

((uint32_t)(strResponseData[12] & 0xFF)<< 9 )

Unless you are on an odd platform where char is not 8 bits (for example some TI DSP's) you probably don't need to pre-mask with 0xff, but it's not hurting anything

Finally it is not 100% clear what you are staring with:

i have an example data [7E FF 01 46 4B CD CC CC CC CC CC 10 40 1B 7E] which is in hex.

Is ambiguous as it is not clear if you mean

[0x7e, 0xff, 0x01, 0x46...]

Which would be an array of byte values which debugging code has printed out in hex for human convenience, or if you actually mean that you something such as

"[7E FF 01 46 .... ]"

Which string of text containing a human readable representation of hex digits as printable characters. In the latter case, you'd first have to convert the character representation of hex digits or octets into into numeric values.

not able to shift hex data in a unsigned long

1 Answers1