Circumference of circle to many decimal places

[Art]

// input ASCII into input array
    // check ASCII numeric range (0x30-0x39 & 0x2E) here, and subtract 0x30 for all but 0x2E
    // rotate input array left until the decimal point is aligned with twopi array

    unsigned int input = 100; // 2:45am cheat
    unsigned char twopi[19] = { 0x00,0x00,0x00,0x06,0x02,0x08,0x03,0x01,0x08,0x05,0x03,0x00,0x07,0x01,0x07,0x09,0x05,0x09,0x00 };
    unsigned char otput[19] = { 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 };
    int adcnt = 0; // counter
    unsigned char dgcnt = 0; // digit index
    unsigned char carry = 0; // carry digit
    while (adcnt < input) { // multiply through addition
        dgcnt = 17; carry = 0;
        while (dgcnt > 0) { // cycle the digits
            otput[dgcnt] = otput[dgcnt] + twopi[dgcnt] + carry; carry = 0;// add digit
            if (otput[dgcnt] > 9) { otput[dgcnt] = otput[dgcnt] - 10; carry = 1; } // carry digit
            dgcnt--;
        } // dgcnt
        if (carry != 0) { otput[dgcnt] = otput[dgcnt] + 1; }
        adcnt++;
    } // adcnt
    twopi[18] = 0x00; // terminate string
    printf("%d%d%d%d.%d%d%d%d%d%d",otput[0],otput[1],otput[2],otput[3],otput[4],otput[5],otput[6],otput[7],otput[8],otput[9]);

        // answer: 1884.9555921538758

[Art]

But in C, since you usually have math.h with floating point, you wouldn’t actually do that.
This was part of a challenge with strict constraints.

To do it in C, it’s:

float circ;
float radius;
const float pi = 3.1415; // or however far you want to take it

radius = 10.0
circ = (2*pi)*radius;

[tumbleweed]

const float pi = 3.1415; // or however far you want to take it

With a single precision float that's about as far as you can take it... 6-7 digits.

You'll almost certainly never get the 17 digit answer.