What's the correct way of using bitfields in C?

Question

I am using bitfields to get easy access on a float library I am trying to make for a microcontroller with no FPU.

The problem is that I can't seem to make it work with bitfields. Take a look:

typedef struct
{
   union{
    unsigned long mantissa: 23;
    unsigned long exponent: 8;
    unsigned long sign: 1;
    float all;

      };

}_float __attribute__((__packed__));

The problem is that when I try to access or change anything it considers the bitfields as 1,8,23 bits from the end respectively. While it should be 23 bits from the end, then 8 bits and then the last bit. Unless I have totally misunderstood the use of bitfields. I thought that using packed would solve the problem but as you can see it didn't.

Any help would be really appreciated. I have been lead to this site while googling more than once so I have high hopes.

The correct way to use bitfields is to not use bitfields. – Carl Norum Jul 27 '11 at 18:25 — Carl Norum, Jul 27 '11 at 18:25

score 18 · Accepted Answer · answered Feb 25 '09 at 19:05

18

You might be missing a struct inside your union.

typedef struct
{
    union{
       struct {
           unsigned long mantissa: 23;
           unsigned long exponent: 8;
           unsigned long sign: 1;
       } float_parts;
       float all;
    };
}_float __attribute__((__packed__));

Note that the order of mantissa/exponent and sign depends one the endianess of the cpu.

answered Feb 25 '09 at 19:05

epatel

45,805
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Thank you for your input but why add a struct inside the union? I made the union of the float's parts with the float to make sure that each bitfield corresponds to the appropriate part of the float. Isn't that enough? – Feb 25 '09 at 19:14
1

a union will make ALL its members share the same memory. I don't think using bitfields will change this behavior, so all your bitfields start at bit 0, just like the "all" float does. Putting them in a struct makes the laid out sequentially. – rmeador Feb 25 '09 at 19:18
You were absolutely right! That's what was needed. I blame my understanding of union then :) . Thank you friend, now I am able to access any part of my float at will. – Feb 25 '09 at 19:26

score 0 · Answer 2 · answered Feb 25 '09 at 19:05

0

The problem is that it is a union. It should be 'struct'.

answered Feb 25 '09 at 19:05

Antti Huima

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score 0 · Answer 3 · answered Feb 25 '09 at 21:39

0

If you are on a glibc platform you can take a look on the ieee754.h header file. It takes care about the endianess stuff. If not it's still probably worth to take a look on it.

answered Feb 25 '09 at 21:39

quinmars

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What's the correct way of using bitfields in C?

3 Answers3

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