Proper Money Handling (JSON-RPC)
Overview
The original bitcoin client stores all bitcoin values as 64-bit integers, with 1 BTC stored as 100,000,000 (one-hundred-thousand of the smallest possible bitcoin unit). Values are expressed as double-precision Numbers in the JSON API, with 1 BTC expressed as 1.00000000
If you are writing software that uses the JSON-RPC interface you need to be aware of possible floating-point conversion issues. You, or the JSON library you are using, should convert amounts to either a fixed-point Decimal representation (with 8 digits after the decimal point) or a 64-bit integer representation.
Improper value handling can lead to embarrassing errors; for example, if you truncate instead of doing proper rounding and your software will display the value "0.1 BTC" as "0.09999999 BTC" (or, worse, "0.09 BTC").
The rest of this page gives sample code for various JSON libraries and programming languages.
JavaScript
The JavaScript Number type is double-precision floating point; JavaScript does not have an integral numeric type.
JavaScript experts: what is best practice? Convert to an integral-like-double, or just do proper rounding on display and do all calculations using doubles?
C/C++
C/C++ JSON libraries return the JavaScript Number type as type 'double'. To convert, without loss of precision, from a double to a 64-bit integer multiply by 100,000,000 and round to the nearest integer:
double dValue = ...from JSON library...;
long long int64Value;
if (dValue > 0) { int64Value = (long long)(dValue * 10e8 + 0.5);
else { int64Value = (long long)(dValue * 10e8 - 0.5);
To convert to a JSON value divide by 100,000,000.0:
double dValue = (double) int64Value / 10e8
Python
Python's Decimal type is perfect for storing Bitcoin values; bitcoin values require 8 digits of precision:
import decimal decimal.setcontext(decimal.Context(prec=8))
Pass the parse_float argument to python's JSON parsing routines to parse JSON values into Decimal:
json.loads(json_string, parse_float=decimal.Decimal)