mpfr precision

[afvallen]

Hi,

I am trying to a basic working program with the precision printed out 
correctly.

mpfr_t t,x,y;
mpfr_init(t);
mpfr_init(x);
mpfr_init(y);
mpfr_set_str(x,"460.8952",10,MPFR_RNDN);
mpfr_set_str(y,"10.0",10,MPFR_RNDN);
mpfr_mul(t,x,y,MPFR_RNDN);
mpfr_printf("%.32Rf",t);


It prints out: 4608.9520000000002255546803968203.

Obviously its not supposed to be that but just "4608.952" instead.

1. Why is that? Other times with for example abs it will just start to 
make up numbers.

2. How can I get sprintf or printed out the right precision if I don't 
know how many numbers there will or could be after the dot, instead of 
just get 32 zeros printed out? The max being 32 however.

3. Why is the calculated output always different from when I use the 
regular OS calculator? It starts to slightly differ the more digits 
after the dot.


Thanks.