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.

No, you can't have an array of bits, as accessed by [ ]

The best you can do is have something like
#define BIT(x) (1<<(x))

Then
for ( i = 0 ; i < n ; i++ ) if ( value & BIT(i) )

Quote:

---

Originally Posted by jaarestad

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:

---

Structures are always padded to a whole number of bytes, sometimes a whole number of words (4 bytes, usually).
So whilst you can declare a strcure with a 1 bit bitfield and then have an array of them, this won't do what you want.
The answer everyone uses is to have an aray of plain unsinged chars, and access functions/macros which use the bitshift and logical operator.

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.