Extract Specific Bits from Array in C

Question

I'm working with a byte-addressed array ParameterArray[20] which has multiple contiguous parameters of varying length:

ParameterA: Start = 0; Length = 2
ParameterB: Start = 2; Length = 1
ParameterC: Start = 3; Length = 6
...

I would like to build a function like so:

int ExtractedData(startBit, endBit, ParameterStart)
{
    return (ParameterArray[ParameterStart] >> endBit) && 2^startBit;
}

The above however wont work if I exceed 8bits, or for example if I want the middle bits of a multi-byte parameter (ie. bits 17:5 of Parameter C)

Any suggestions on how to implement this?

Edit: I've modified my function as follows but I'm still limited by the size of int with this implementation:

int ExtractedData(startBit, endBit, ParameterStart, parameterLength)
{
    int tempVar = 0;

    for (int i=0; i < parameterLength; i++)  {
         tempVar = (tempVar << 8 | ParameterArray[ParameterStart+ i]; 
    }    
    return (tempVar >> endBit) & ((1 << (startBit - endBit + 1)) - 1);
}

Paul R · Accepted Answer · 2014-03-07 00:20:56Z

1

Two mistakes, I think. Change:

return (ParameterArray[ParameterStart] >> endBit) && 2^startBit;

to:

return (ParameterArray[ParameterStart] >> endBit) & ((1 << (endBit - startBit)) - 1);
                                                  ^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                                  |                |
   need bitwise AND here (not logical AND) --------      need a suitable mask here

Note that ^ is the bitwise XOR operator in C and related languages, it is not an exponentiation operator.

answered Mar 7, 2014 at 0:20

Paul R

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4 Comments

spizzak Over a year ago

I fixed those two errors you mention but do you have any suggestions on how to expand this to work with multi-byte parameters?

Paul R Over a year ago

What is the maximum width in bits that you need to return ? E.g. would it always fit in a 32 bit int ?

spizzak Over a year ago

I've made the modification in order for it to be able to handle up to a 32 bit int but I'm not sure how efficient it is. I also need to return up to 6 bytes sometimes but ultimately I could work around that issue if there is no better way to deal with it.

Paul R Over a year ago

Well if you need up to 6 bytes then use a suitable 8 byte data type, such as uint64_t.

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