3.3.4.2 How Can I Access Individual Bits of a Variable?

There are several ways of doing this. The simplest and most portable way is to define an integer variable and use macros to read, set, or clear the bits within the variable using a mask value and logical operations, such as the following.

#define  testbit(var, bit)   (!!(var) & (1 <<(bit)))
#define  setbit(var, bit)    ((var) |= (1 << (bit)))
#define  clrbit(var, bit)    ((var) &= ~(1 << (bit)))

These, respectively, test to see if bit number, bit, in the integer, var, is set; set the corresponding bit in var; and clear the corresponding bit in var. Alternatively, a union of an integer variable and a structure with bit-fields (see 8.5.2 Bit Fields in Structures) can be defined, for example,

union both {
   unsigned char byte;
   struct {
      unsigned b0:1, b1:1, b2:1, b3:1, b4:1, b5:1, b6:1, b7:1;
   } bitv;
} var;

This allows you to access byte as a whole (using var.byte), or any bit within that variable independently (using var.bitv.b0 through var.bitv.b7).