TIP: Using Unions for Manipulating Bits

This is simple custom data type that allows you to manipulate bits, bytes, or whatever comes into a 32 bit integer. It uses unions for this purpose because all data types within a union share the same memory. This can be very useful for low level purposes such as setting flags without using masking.

Here is the custom type union comprising one DWORD.

typedef struct _BITS8
{
      unsigned char bit0 : 1;
      unsigned char bit1 : 1;
      unsigned char bit2 : 1;
      unsigned char bit3 : 1;
      unsigned char bit4 : 1;
      unsigned char bit5 : 1;
      unsigned char bit6 : 1;
      unsigned char bit7 : 1;
}BITS8;

typedef struct _QUATERS
{
      unsigned char quater0 : 4;
      unsigned char quater1 : 4;
}QUATERS;


typedef	union   _BITSQUATERS
{
      BITS8 bits;
      QUATERS quaters;
      unsigned char byte;
}BITSQUATERS;

typedef union  _BITS32
{

      //all 4 bytes - represented as 8 bits and 2 quaters
      BITSQUATERS byte[4];
      short word[2]; //two 16 bit int
      int dword;     //32 bit int

}BITS32;

The structure BITS8 represents single byte as 8 separate bits you can access! If you have a variable of type char and you wish to change some of the bits within it, then all you have to do is take its address, cast it to BITS8 * and then you can change whichever bit you like.

      char c = 0;
      BITS8 *pBits8 = (BITS8  *)&c;
      pBits8->bit4 = 1;//set bit4 of variable c

If you would want to change the the high an low quater of varialble c, then cast its address to QUATERS * just like this:

      char c=0;
      QUATERS *pQuaters=(QUATERS *)&c;
      pQuaters->quater0 = 0xC; //set the low quater of c
      pQuaters->quater1 = 0xD; //set the high quater of c

If you want to set bits, quaters or the whole byte, then use the BITSQUTERS * pointer this way:

      char c=0;
      BITSQUATERS *pBitsQuaters =(BITSQUATERS *)&c;
      pBitsQuaters->byte = 0xFF;           //set the whole byte
      pBitsQuaters->quaters.quater0 = 0xC; //set the low quater of c
      pBitsQuaters->quaters.quater1 = 0xD; //set the high quater of c
      pBitsQuaters->bits.bit7 = 0;         //set bit7

Here is how to set up dword, low and high word, separate bytes and bits as well — using a BITS32 * pointer:

      int i = 0;
      BITS32 * pbits32 = (BITS32 *) &i;
      pbits32->dword = 0x33221100;     	//set all 32 bits
      pbits32->word[0] = 0xcc11;          //set low int16 of dword
      pbits32->byte[0].byte = 0xff;   //set first byte
      pbits32->byte[0].quaters.quater0 = 0xF;//set first byte low quarter
      pbits32->byte[3].bits.bit7 = 1; //set third byte bit7

Hope that will come in handy! Enjoy