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-million 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 original bitcoin client does proper, full-precision rounding for all values passed to it via the RPC interface. So, for example, if the value 0.1 is converted to the value "0.099999999999" by your JSON-RPC library, that value will be rounded to the nearest 0.00000001 bitcoin and will be treated as exactly 0.1 bitcoins.

The rest of this page gives sample code for various JSON libraries and programming languages.

ECMAScript

function JSONtoAmount(value) {
    return amount = Math.round(1e8 * value);
}

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:

int64_t JSONtoAmount(double value) {
    return (int64_t)(value * 1e8 + (value < 0.0 ? -.5 : .5));
}

To convert to a JSON value divide by 100,000,000.0, and make sure your JSON implementation outputs doubles with 8 or more digits after the decimal point:

 double forJSON = (double)amount / 1e8;

Python

If you are using python-json-rpc, you should convert its floating point values to and from the python Decimal type, specifying 8 digits of precision. For example:

 import decimal
 from jsonrpc import ServiceProxy

 access = ServiceProxy("http://user:password@127.0.0.1:8332")
 info = access.getinfo()
 decimal.setcontext(decimal.Context(prec=8))
 balance = decimal.Decimal("%.8f"%info['balance'])
 amount_to_send = balance / decimal.Decimal('2')
 access.sendtoaddress('...bitcoin address...', float(amount_to_send))

If you are using the standard json library, you can use the parse_float arguments to python's JSON parsing routines to parse JSON values into Decimal and use the DecimalEncoder class to write out Decimal values:

 import decimal
 import json
 
 # From http://stackoverflow.com/questions/1960516/python-json-serialize-a-decimal-object
 class DecimalEncoder(json.JSONEncoder):
   def _iterencode(self, o, markers=None):
     if isinstance(o, decimal.Decimal):
       return (str(o) for o in [o])
     return super(DecimalEncoder, self)._iterencode(o, markers)
 
 decimal.setcontext(decimal.Context(prec=8))
 print json.dumps(decimal.Decimal('10.001'), cls=DecimalEncoder)
 print json.dumps({ "decimal" : decimal.Decimal('1.1'), "float" : 1.1, "string" : "1.1" }, cls=DecimalEncoder)
 print json.loads('{"amount": 0.333331}', parse_float=decimal.Decimal)

Output is:

 10.001
 {"decimal": 1.1, "float": 1.1000000000000001, "string": "1.1"}
 {u'amount': Decimal('0.333331')}