Unable to save a float value to a bitfield structure

Question

I have a structure

struct {
   u32 var1 :7;
   u32 var2 :4;
   u32 var3 :4;
   u32 var4 :1;
   u32 var5 :4;
   u32 var6 :7;
   u32 var7 :4;
   u32 var8 :1;
          


        

Answer 1

You can't store a double (64 bits) into a field of only 4 bits. You can either do it this way:

struct {
   u32 var1 :7;
   u32 var2 :4;
   u32 var3 :4;
   u32 var4 :1;
   double var5;
   u32 var6 :7;
   u32 var7 :4;
   u32 var8 :1;
        } my_struct; 

or this way

struct {
   u32 var1 :7;
   u32 var2 :4;
   u32 var3 :4;
   u32 var4 :1;
   u64 var5 :64;
   u32 var6 :7;
   u32 var7 :4;
   u32 var8 :1;
        } my_struct; 
  ..

  struct1[i].var5 = *(u64*)&x; // reinterpret the double as a memory array of 8 bytes

The second way is not recommended.

If you want to store 64 bits in 4 bits, read up on how floating point (IEEE) works. http://en.wikipedia.org/wiki/IEEE_floating_point