Adding proposed fix, with tests, resolves #792

include/boost/spirit/home/karma/numeric/real_policies.hpp

@@ -257,8 +257,20 @@ namespace boost { namespace spirit { namespace karma
             //    generate(sink, right_align(precision, '0')[ulong], n);
             // but it's spelled out to avoid inter-modular dependencies.
 
-            typename remove_const<T>::type digits = 
-                (traits::test_zero(n) ? 1 : ceil(log10(n + T(1.))));
+	    unsigned int digits=1; //should be number of digits n(truncating any fraction)
+	    if(!boost::spirit::traits::test_zero(n)) {
+	      static constexpr uint64_t limit = UINT64_MAX / 10;
+	      const T num = floor(n);
+	      for (uint64_t x = 10u, i = 1u;; x *= 10, i++) {
+		if (num < x) {
+		  digits=i;break;
+		}
+		if (x > limit) {
+		  digits= i + 1;break;
+		}
+	      }
+	    }
+
             bool r = true;
             for (/**/; r && digits < precision_; digits = digits + 1)
                 r = char_inserter<>::call(sink, '0');

test/karma/real3.cpp

@@ -198,6 +198,16 @@ int main()
             ));
     }
 
+    // test for #792: off by a magnitude due to fraction_part
+    // notice that it applies to all forms of fractions
+    // that end with a sequence of 0.009999
+    {
+        BOOST_TEST(test("1.09e-06",
+			double_,
+            1.09e-06
+            ));
+    }
+
     return boost::report_errors();
 }