Question

Doing some experiences to understand bit fields

Hi have this code:

01  #include <stdio.h>
02  #include <stdlib.h>
03
04  void AddToBitfield(int *bitfield, int bitCount, int value);
05  int ReadFromBitfield(int *bitfield, int bitCount);
06
07  int main(){
08      /*Device list (0 Modem,1 Keyboard,2 Mouse,3 Speakers,4 Joystick,5 Flash Drive,6 Scanner,7 Printer,8 Microphone,9 Webcam,10 Monitor)*/
09      int device=0,memLoc=0,data=0;
10      int number = 0;
11
12      memLoc = 01; /*put 01 or 10*/
13      device = 15; /*Device id*/
14      data = 12343; /*Data to store - Only less or equal then 65535*/
15
16      AddToBitfield(&number,4,device);
17      AddToBitfield(&number,16,data);
18      AddToBitfield(&number,2,memLoc);
19
20      printf("--%d---\n",number);
21
22      printf("Memory location: %d\n",ReadFromBitfield(&number,2));
23      printf("Data stored: %d\n",ReadFromBitfield(&number,16));
24      printf("Device: %d\n",ReadFromBitfield(&number,4));
25
26      return 0;
27  }
28
29  void AddToBitfield(int *bitfield, int bitCount, int value){
30      *bitfield <<= bitCount;
31      *bitfield |= value;
32  }
33
34  int ReadFromBitfield(int *bitfield, int bitCount){
35      int value = *bitfield & ((1 << bitCount) - 1);
36      *bitfield >>= bitCount;
37
38      return value;
39  }

Using data = 12343; and with memLoc = 01; or memLoc = 10; the printf will show all as expected.

Using data = 12346; and with memLoc = 01; or memLoc = 10; its the same thing. The printf will show all as expected.

But if I use data = 12344; or data = 12345; if I use memLoc = 01; the print will show what is expected in both cases, but if i use memLoc = 10; on the first case it will print Data stored: 12346 and on the second case Data stored: 12347.

Why is this happening?

Answer 1

You can use this C struct for reading/writing bit fields:

struct MyType
{
    uint16_t memLoc:4;
    uint16_t device:4;
    uint16_t _reserved:8;
    uint16_t data;
};

Your code will looks like this:

int main()
{
    // ...

    struct MyType number;
    uint32_t numberInt;

    number.memLoc = 1;
    number.device = 15;
    number.data = 12343;

    memcpy(&numberInt, &number, sizeof(struct MyType));
    printf("--%d---\n", numberInt);

    printf("Memory location: %d\n", number.memLoc);
    printf("Data stored: %d\n", number.data);
    printf("Device: %d\n", number.device);

    // ...
}

Tell me how many bits are in each field, and i’ll fix that structure for you.

Here is an article about bit fields: http://en.wikipedia.org/wiki/Bit_field