C Language Bit-fields When are bit-fields useful?


Example

A bit-field is used to club together many variables into one object, similar to a structure. This allows for reduced memory usage and is especially useful in an embedded environment.

 e.g. consider the following variables having the ranges as given below.
 a --> range 0 - 3
 b --> range 0 - 1
 c --> range 0 - 7
 d --> range 0 - 1
 e --> range 0 - 1

If we declare these variables separately, then each has to be at least an 8-bit integer and the total space required will be 5 bytes. Moreover the variables will not use the entire range of an 8 bit unsigned integer (0-255). Here we can use bit-fields.

typedef struct {
   unsigned int a:2;
   unsigned int b:1;
   unsigned int c:3;
   unsigned int d:1;
   unsigned int e:1;
} bit_a;

The bit-fields in the structure are accessed the same as any other structure. The programmer needs to take care that the variables are written in range. If out of range the behaviour is undefined.

int main(void)
{
   bit_a bita_var;
   bita_var.a = 2;              // to write into element a
   printf ("%d",bita_var.a);    // to read from element a.
   return 0;
}

Often the programmer wants to zero the set of bit-fields. This can be done element by element, but there is second method. Simply create a union of the structure above with an unsigned type that is greater than, or equal to, the size of the structure. Then the entire set of bit-fields may be zeroed by zeroing this unsigned integer.

typedef union {
    struct {
       unsigned int a:2;
       unsigned int b:1;
       unsigned int c:3;
       unsigned int d:1;
       unsigned int e:1;
    };
    uint8_t data;
} union_bit;    

Usage is as follows

int main(void)
{
   union_bit un_bit;
   un_bit.data = 0x00;        // clear the whole bit-field
   un_bit.a = 2;              // write into element a
   printf ("%d",un_bit.a);    // read from element a.
   return 0;
}

In conclusion, bit-fields are commonly used in memory constrained situations where you have a lot of variables which can take on limited ranges.