Grabbing n bits from a byte

Question

I\'m having a little trouble grabbing n bits from a byte.

I have an unsigned integer. Let\'s say our number in hex is 0x2A, which is 42 in decimal. In binary it looks li

Answer 1

You could use bitfields for this. Bitfields are special structs where you can specify variables in bits.

typedef struct {
  unsigned char a:5;
  unsigned char b:3;
} my_bit_t;

unsigned char c = 0x42;
my_bit_t * n = &c;
int first = n->a;
int sec = n->b;

Bit fields are described in more detail at http://www.cs.cf.ac.uk/Dave/C/node13.html#SECTION001320000000000000000

The charm of bit fields is, that you do not have to deal with shift operators etc. The notation is quite easy. As always with manipulating bits there is a portability issue.