gistfile1.txt

#include <boost/spirit/include/classic_core.hpp>
#include <iostream>
#include <string>
#include <stdio.h>
#include <stdint.h>

using namespace std;
using namespace boost::spirit::classic;

struct count_satoshis
{
    count_satoshis(uint64_t& val_) : val(val_) {}

    void operator()(const char* a, const char* b) const
    {
        val = 0;
        int i, c;
        for (i = 0; i < 8; i++) {
            if (a + i < b)
                c = a[i] - '0';
            else
                c = 0;
            val *= 10;
            val += c;
        }
    }

    uint64_t& val;
};

uint_parser<uint64_t, 10, 1, -1> uint_left;
uint_parser<uint64_t, 10, 1, 1> digit;

bool parse_number(char const* str, int64_t& n)
{
    bool is_negative = false;
    uint64_t left = 0, right = 0;
    count_satoshis save_right(right);

    rule<>
        left_ = uint_left[assign_a(left)],
        right_ = (+digit)[count_satoshis(right)],
        number_ = !sign_p[assign_a(is_negative)] >> ((!left_) >>  ch_p('.') >> right_
                                                    |  left_  >> !ch_p('.'));
   
    bool ret = parse(str, number_).full;

    if (ret)
        n = (is_negative ? -1 : 1) * (left * 100000000 + right);

    return ret;
}

int main()
{
    string str;
    int64_t n;
    while (getline(cin, str))
        if (parse_number(str.c_str(), n))
            cout << str << " --> " << n << endl;
        else
            cout << "failed" << endl;

    return 0;
}

Some inputs and outputs:

$ ./a.out
> 123.4567890123
123.4567890123 --> 12345678901

> 123456789.12345678
123456789.12345678 --> 12345678912345678

> 123456789.12345677
123456789.12345677 --> 12345678912345677

> 123456789.12345676
123456789.12345676 --> 12345678912345676

> -1.1
-1.1 --> -110000000

> -0.1
-0.1 --> -10000000

> -.1
-.1 --> -10000000

> -1.
-1. --> -100000000

> 1.1
1.1 --> 110000000

> 0.1
0.1 --> 10000000

> .1
.1 --> 10000000

> 1.
1. --> 100000000

> 0.
0. --> 0

> .0
.0 --> 0

> 0.0
0.0 --> 0

> .
failed

> 12..34
failed

> 1234
1234 --> 123400000000

> 1234567890
1234567890 --> 123456789000000000

> 12345678901
12345678901 --> 1234567890100000000

> 123456789012
123456789012 --> -6101065172509551616