Get floating/decimal portion of a float

I would like:

unknown_function(123.456) -> 456
unknown_function(1234.56) -> 56

Or

unknown_function(123.456) -> "456"

Is there a builtin for this? The builtin trunc/1 does the opposite:

2> trunc(123.456).
123

There is this answer for C: Extract decimal part from a floating point number in C and this for Java: How to get the decimal part of a float?