use strict;
use warnings;
use Math::MPFR qw(:mpfr);

my $prec_53   = Rmpfr_init2(53);
my $prec_4000 = Rmpfr_init2(4000);

my $d = 1.3;

Rmpfr_set_d($prec_53, $d, MPFR_RNDN);
# The 53 bits of $prec_53 are now set to:
# 10100110011001100110011001100110011001100110011001101

Rmpfr_set_d($prec_4000, $d, MPFR_RNDN);
# The first 53 bits are as for $prec_53
# The remaining 3947 bits are 0.

# But we'll run a check:

die "Things aint right"
 if $prec_53 != $prec_4000;

print "$prec_53\n";
# prints 1.3

print "$prec_4000\n";
# prints 1.3000000000000000444089209850062616169452667236328125