No Znx!

I was further discussing C ideas and this time we came across bit flags (also called bit fields). The idea behind them is conservation of memory, a boolean data type consumes 1 byte of memory but all that is really needed is a single bit. Therefore you can store several booleans in a single byte.

Here is a simple example, note that each has a specific value to ensure that each matches a single bit in the 8-bit variable:

enum options {
  option_a = 0x01,  /*   1 == 00000001 */
  option_b = 0x02,  /*   2 == 00000010 */
  option_c = 0x04,  /*   4 == 00000100 */
  option_d = 0x08,  /*   8 == 00001000 */
  option_e = 0x10,  /*  16 == 00010000 */
  option_f = 0x20,  /*  32 == 00100000 */
  option_g = 0x40,  /*  64 == 01000000 */
  option_h = 0x80,  /* 128 == 10000000 */
};

So basically we have 8 boolean flags (0 off/false, 1 on/true). We can use these by utilising bitwise operations. There is really only 3 operations that you will use regularly on bit flags so lets see them.

Switching individual/multiple options “on” (or setting value true):

options |= option_a;
/* Translates to:
  options = options | option_a;
  options = 00000000 | 00000001;
  options = 00000001;
*/
 
options = option_a | option_c | option_h;
/* Translates to:
  options = 00000001 | 00000100 | 10000000;
  options = 10000101;
*/

Equally switching options “off” (or setting false) can be done as follows (note we are using the options from above, thus a, c and h are on):

options &= ~option_a;
/* Translates to:
  options = options & ~option_a;
  options = 10000101 & ~00000001;
  options = 10000101 & 11111110;
  options = 10000100;
*/

Obviously you need a basic grasp of boolean mathematics to be able to handle this but learning the basics shouldn’t be too hard. Finally the last thing to do is check to see if a flag is “on” or “off” (again using the options from before, so c and h are “on”:

if( options & option_b ) printf("B is ON\n");
/* 10000100 & 00000010 = 00000000 .. i.e. false, so no output */
if( options & option_c ) printf("C is ON\n");
/* 10000100 & 00000100 = 00000100 .. i.e. true, so "C is ON" is outputting */

As with most things in C, once you start using and abusing it should start to fall into place. I hope that this serves as a basic introduction to bit fields.