Charts: ChartUtils Crash on 32 Bit Devices

[ObjColumnist]

On 32 Bit Devices the function decimals in ChartUtils causes a bad memory access when the default formatter for a Chart is created.

    internal class func decimals(_ number: Double) -> Int
    {
        if number.isNaN || number.isInfinite || number == 0.0
        {
            return 0
        }

        let i = roundToNextSignificant(number: Double(number))

        if i.isInfinite
        {
            return 0
        }

        return Int(ceil(-log10(i))) + 2
    }

That is because i is NaN so the line return Int(ceil(-log10(i))) + 2 crashes.

[liuxuan30]

need more info - why it is NaN in the first place?

[danielgindi]

Problem is I have no 32bit device to test on. I don't see why would the value end up being NaN just because of 32 bit device.
What's the input number in your case?

[ObjColumnist]

It happens when the chart creates its default formatter.

It was a huge number though, which might be an issue in itself.

I can check next time have access to a 32 Bit device (later in the week).

[jimmyjeng]

i run on iPad4

NSMutableArray *dataSets = [[NSMutableArray alloc] init];
LineChartData *data = [[LineChartData alloc] initWithDataSets:dataSets];
_chartView.data = data;

Then , the var magnitude in fun roundToNextSignificant would be 0 , so i will be NaN.

[liuxuan30]

would you post more vars that lead magnitude to be 0? We don't know what your data entries are. And not giving enough info

[jimmyjeng]

i reproduce this crash in ChartsDemo by del one line in fun setDataCount of LineChart1ViewController.m .

        NSMutableArray *dataSets = [[NSMutableArray alloc] init];
//        [dataSets addObject:set1];
        LineChartData *data = [[LineChartData alloc] initWithDataSets:dataSets];
        _chartView.data = data;

then , the var pw Int -308 would lead magnitude to be 0 on iPad4.
let magnitude = pow(Double(10.0), Double(pw))

[spalmen]

I just encountered this crash today, in Charts 3, on an 9.2.1 iPhone 4S device, when ChartData contains an empty DataSet.
let lineChartYDataSet = LineChartDataSet(values: nil, label: nil)
lineChartView.data = LineChartData(dataSets: [ lineChartYDataSet ])

ChartViewBase.data setter:
// calculate how many digits are needed
setupDefaultFormatter(min: _data!.getYMin(), max: _data!.getYMax())

Because ChartData's DataSet is empty, its yMin and yMax are still DBL_MAX and -DBL_MAX, respectively.
ChartViewBase.setupDefaultFormatter(min: DBL_MAX, max: -DBL_MAX)
ChartData.entryCount is 0, so 'reference' is set to DBL_MAX.
reference = absMin > absMax ? absMin : absMax
. . .
// setup the formatter with a new number of digits
let digits = ChartUtils.decimals(reference)

ChartUtils.decimals(_ number: DBL_MAX)
'number' isn't NaN, or Infinite, or 0 so it evades the sanity check:
let i = roundToNextSignificant(number: Double(number))

ChartUtils.roundToNextSignificant(number: DBL_MAX) // number: 1.7976931348623157E+308
let d = ceil(log10(number < 0.0 ? -number : number)) // d: 309
let pw = 1 - Int(d) // pw: -308
let magnitude = pow(Double(10.0), Double(pw)) // magnitude: 0 <-- uhoh
let shifted = round(number * magnitude) // shifted: 0
return shifted / magnitude // NaN

Back in ChartUtils.decimals
if i.isInfinite // i: NaN
{
return 0
}
return Int(ceil(-log10(i))) + 2
fatal error: Double value cannot be converted to Int because it is either infinite or NaN

Interestingly, on a 9.3.5 iPhone 5s device (64-bit) _and_ the Xcode 8.1 iPhone 4S (9.2) simulator, pow() with the same inputs returns a different result:
let magnitude = pow(Double(10.0), Double(pw)) // pw: -308, magnitude: 9.999999999999999E-309 <-- Abby Subnormal?
let shifted = round(number * magnitude) // shifted: 2
return shifted / magnitude // +Inf

But ChartUtils.decimals' 'if i.isInfinite' saves the day.

Unfortunately, I don't have a 32-bit device running iOS 10 to see how pow() behaves here.

As a hacky workaround, I found that adding a dummy DataEntry(x:0, y:0) to the DataSet (only public way I could find to set yMin and yMax to 0), then clearing the DataSet to remove it, _before_ setting ChartView.data, avoids the crash.

(I also have to fuss with the LineChartView.xAxis to suppress the labels the axis renderer so helpfully wants to display for a empty dataset, but my usage is probably atypical. The chart displays realtime data and at this point, before I've collected any data points, I don't want any xAxis labels.)

I'm a Charts nooB, so I won't embarrass myself by suggesting where to guard against this DBL_MAX issue. :) Hope this helps…

[liuxuan30]

Thanks for the info! Looks like the 32bit device has troubles.. @danielgindi

[brunce]

I stumbled on this issue yesterday when creating and assigning an empty ChartData:

LineChartData *chartData = (LineChartData *)chartView.data;        
if (!chartData) {
    chartData = [[LineChartData alloc] init];
    chartView.data = chartData;
}

// create dataSets and add to chartData 

But when adding dataSets before assigning to chartData, everything works fine:

LineChartData *chartData = (LineChartData *)chartView.data;
if (!chartData) {
    chartData = [[LineChartData alloc] init];
}

// create dataSets and add to chartData

chartView.data = chartData;

Only crashed on the 32 bit, not on the 64 bit device.
Charts 3.0.0, iPad3, iOS 9.2

[liuxuan30]

@brunce make sure you call chartView.data = chartData at last, because the data setter will invoke notifyDataSetChanged() and involve new calculation about the contentRect, axis property, matrix, etc. Setting chartData wildly would cause unexpected issues.

@ObjColumnist I think you made the same mistake. You shouldn't feed data when you only have empty array. Everything is like default DBL_MAX /DBL_MIN and on 64 devices, the i is then +inf, while you say it's NaN on 32 bit device. I will try to find a 32 bit device to test

[liuxuan30]

Alright, I found an iPhone 5 to test, it's indeed because the CPU arch, same as @spalmen:
32 bit:

(lldb) po pow(10.0, -308.0)
0.0

64bit:

(lldb) po pow(10.0, -308.0)
9.9999999999999991e-309

Then, due to the difference, shifted will be 2.0 on 64 bit, but 0 on 32 bit:
64 bit

(lldb) po number
1.7976931348623157e+308

(lldb) po magnitude
9.9999999999999991e-309

(lldb) po shifted
2.0

(lldb) po shifted/magnitude
inf

32 bit

(lldb) po number
1.7976931348623157e+308

(lldb) po magnitude
0.0

(lldb) po shifted
0.0

(lldb) po shifted/magnitude
nan

@danielgindi I will merge that PR. I tested it on iPhone 5, should be good.

[acegreen]

This is [still] present on 64bit devices (iPhone 6S plus)

[liuxuan30]

@acegreen please give proofs how this is still present like I tested above.