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; 

typedef struct _QUATERS
      unsigned char quater0 : 4; 
      unsigned char quater1 : 4; 

typedef	union   _BITSQUATERS
      BITS8 bits;
      QUATERS quaters;
      unsigned char byte;

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


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

bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
The way struct BITS8 represents one byte in memory

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

quater1 quater0
The way struct QUATERS represents one byte in memory

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

quater1 quater0
bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
The way union BITSQUATERS represents one byte in memory

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 

byte3 byte2 byte1 byte0
quater1 quater0 quater1 quater0 quater1 quater0 quater1 quater0
bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
The way union BITS32 represents one double word in memory

Hope that will come in handy! Enjoy



  • Endian considerations

    Posted by foxmuldr on 02/24/2010 01:02am

    Depending on the machine, big-endian or little-endian compatibility issues could also arise. As long as you're dealing with a single machine it should be okay, but if you're designing code to interact with other systems it will likely break.

  • Just be aware of this

    Posted by S_M_A on 07/02/2009 03:34pm

    The packing order for bitfields is not standardized i.e. this type of code is not very portable. To further complicate things, using unsigned chars in bitfields isn't according to the standard either so changing compiler might also make the code break.

  • Some code examples would be nice to have

    Posted by CBasicNet on 06/09/2009 09:51pm

    Hi bat_lubo2001,
    Thanks for this useful tip. Some code examples on how to access the members of this complex union would be very nice to have for beginners.

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Being first and fast to market is critical for success. In today's hyper-competitive environment, driving innovation to stay ahead of the competition relies on the ability to seize new opportunities to develop IT solutions faster and more efficiently. This Executive Brief discusses how organizations can meet this high-pressure challenge by moving development of enterprise applications to the cloud, leveraging Platform-as-a-Service (PaaS) and in the process reducing costs, avoiding complexity, and increasing …

  • Thanks to the Internet of Things (IoT), physical assets are turning into participants in real-time global digital markets. The countless types of assets around us will become as easily indexed, searched and traded as any online commodity. While some industries will be tougher to transform than others – those with physical limitations, such as manufacturing, will be harder to digitize – untold economic opportunities exist for growth and advancement. Our research shows this will create a new "Economy …

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date