Thread: Creating a bitfield array in C

Hi, I am curious to know if there is a way that one could create a bitfield using the standard technique of creating a structure within a union, as follows:

Code:

typedef union {
      struct {
         unsigned b0 : 1;
         unsigned b1 : 1;
               :
               :
         unsigned b(n-1) : 1;
      } bits;
      unsigned int value;
  } BIT_FIELD_TYPE;

Except, what I'd like to do is to replace all the single-bit elements in the bits structure with a single statement that creates an array of, say, 32 values. The clear advantage of this is that it could be traversed using an iterator, ...

Code:

   main() {
      BIT_FIELD_TYPE foo;
      unsigned int i;

      ...
      for (i = 0; i < n; i++) {
         ... (print out foo.bits.b[i]) ...
      }

So far, I've not figured out a way to do it, either as an array, or using a pointer to iterate through the individual bits.

I'd love to hear if anyone has a solution to this - or even just comments telling me I'm nuts.

Thanks.

You need to be more specific about what you want to do. This is a terrible way to print a non-negative integer value in binary notation, for instance.

I am trying to implement a data structure that implements a generic linear feedback shift register (LFSR) of variable length and with a separate coefficient vector (another 32-bit value that identifies which tap points in the LFSR participate in the feedback structure).

Thank you, all, for your input. (I agree that if all I wanted to do was print out the binary representation, I'd never consider something like this).

I've settled on this:

Code:

#define BIT(x,b) ((x & (1<<b)) >> b)

This works very well for what I want.

Code:

 struct BoxBits
  {
   unsigned int Bit1 :1;
   unsigned int Bit2 :1;
   unsigned int Bit3 :1;
   unsigned int Bit4 :1;
   unsigned int Bit5 :1;
   unsigned int Bit6 :1;
   unsigned int Bit7 :1;
   unsigned int Bit8 :1;
  };
 union placeholder
  {
   struct BoxBits Bits;
   unsigned int Word;
  };
 union placeholder Boxes[39];

this is what i used to make 40 word<->bit structs

if i say

Code:

Boxes[0].Bits.Bit1=8

then box 0 bit 8 is 1 (so word would be 128)

if i say

Code:

Boxes[39].Word =255

then box 40 the bits would be 11111111

that way i can send and poll in decimal, or send and poll one bit at a time

seems though i can put box into an array, but cant get the bis into an array within the box array! i must be missing something!

unioning bitfields with other types is undefined behavior. c++ - Bitfield manipulation in C - Stack Overflow. see the reference to the C standard.