yet another segfault - array out of bound is not caught by try catch

Sep 17, 2021

seany

Sep 17, 2021

Nicholas Wilson

Sep 17, 2021

jfondren

Sep 18, 2021

russhy

September 17, 2021

yet another segfault - array out of bound is not caught by try catch

Posted by seany

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seany

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I have now this function, as a private member in a Class :

    double calculate_lineLength( int i)  {
        field.rawData [] * rd;                   // ignore the details, this works;
        rd = cast (field.rawData [] *)  dataSet; // ignore the details, this works;

        auto l = this.allLines[i];               // this is defined as
                                                 // int [][] allLines =
                                                 //            new int [][] (0,0)
                                                 // in case of failure, this is empty
        double r = 0;
        try {
            writeln("l ", l);                    // prints []
            writeln("i ", i);                    // prints i, in this case, i is set to 0
            writeln(l.length);                   // prints 0
            writeln("l0 ", l[0]);                // segfault - i want it to be caught
            write("will print");
            writeln("rd", (*rd));
            write("will not print...");
            auto p0 = (*rd)[l[0]];
            auto p1 = (*rd)[l[1]];
	    r = calculate_geoDistance_vincenty(p0.lat,p1.lat, p0.lon, p1.lon);
        } catch (RangeError er) {
            writeln("range error");
        }
            return r;
    }

Compile with dub build --compiler=ldc2 . this should enable array bound checking options.

I am debugging with gdb :

gdb ./myprogram

Then, in gdb console :

run arg1 arg2

Result is :

    91753
    91754
    91755
    91756
    [New Thread 0x7ffff7560640 (LWP 45344)]
    [New Thread 0x7fffeffff640 (LWP 45345)]
    [New Thread 0x7ffff6d5f640 (LWP 45346)]
    [New Thread 0x7ffff655e640 (LWP 45347)]
    [New Thread 0x7ffff5d5d640 (LWP 45348)]
    [New Thread 0x7ffff555c640 (LWP 45349)]
    [New Thread 0x7ffff4d5b640 (LWP 45350)]
    [New Thread 0x7fffef7fe640 (LWP 45351)]
    [New Thread 0x7fffeeffd640 (LWP 45352)]
    [New Thread 0x7fffee7fc640 (LWP 45353)]
    [New Thread 0x7fffedffb640 (LWP 45354)]
    [New Thread 0x7fffed7fa640 (LWP 45355)]
    [New Thread 0x7fffecff9640 (LWP 45356)]
    [New Thread 0x7fffbffff640 (LWP 45357)]
    [New Thread 0x7fffbf7fe640 (LWP 45358)]
    [New Thread 0x7fffbeffd640 (LWP 45359)]
    [New Thread 0x7fffbe7fc640 (LWP 45360)]
    [New Thread 0x7fffbdffb640 (LWP 45361)]
    [New Thread 0x7fffbd7fa640 (LWP 45362)]
    getting LINES done
    alllines: []
    l []
    i 0
    0
    Thread 1 "myprogram" received signal SIGSEGV, Segmentation fault.
    _D14analysisEngine9geoEngine20calculate_lineLengthMFiZd (this=<optimized out>, i=0) at source/analysisEngine.d:15526
    15526
    writeln("l0 ", l[0]);

So, to see what is going on, i use the command bt:

    #0  _D14analysisEngine9geoEngine20calculate_lineLengthMFiZd (this=<optimized out>, i=0) at source/analysisEngine.d:15526
    #1  0x00005555555dba40 in _D14analysisEngine9geoEngine13add_turnLinesMFZv (this=<optimized out>) at source/analysisEngine.d:7387
    #2  0x00005555555e375a in _D14analysisEngine9geoEngine15analyze_tillageMFZv (this=0x7ffff756a000) at source/analysisEngine.d:5329
    #3  0x000055555560d082 in _Dmain (args=...) at source/AI.d:123

Okey, I know where to look for : it's the line asking for writeln("l0 ", l[0]);.

But should it not be caught by range error ? If I do `print l`in gdb, i find :
    $1 = {length = 0, ptr = 0x0}

With print l[0] i get: Attempt to take address of value not located in memory.. I believe the array printing syntax is valid; see here.

What absolute rookie mistake am I committing? What does it mean : "Attempt to take address of value not located in memory" ? I am not even calling / accessing a pointer. I am trying to extract a value outside an array bound.

I imagine they all have their addresses. But with the bound checking operation in place, would the bound error be triggered before the attempt to take unavailable address error has a chance to trigger?

Thank you.

On Friday, 17 September 2021 at 11:10:33 UTC, seany wrote:

Compile with dub build --compiler=ldc2 . this should enable array bound checking options.

By default, yes. run dub -v build --compiler=ldc2 to see the exact commands that dub runs.

But should it not be caught by range error ?

Based on what you've said, yes it should.

If I do print lin gdb, i find :
$1 = {length = 0, ptr = 0x0}
With print l[0] i get: Attempt to take address of value not located in memory..

i.e., a segfault. null (or 0x0 (or 0)) isn't part of the memory addresses your program is allowed to access, so memory protection prevents the attempt to access it.

What absolute rookie mistake am I committing?

From this it doesn't sound like you are committing one, but if you're wanting bounds checking to be a normal part of program logic, and not something that only ever happens due to a programmer's error, then I think you're cutting against the grain of the language, where

bounds checking is easily removed from all but @safe functions with normal flags
flags exist to remove it from @safe functions also
the *Error family of exceptions, including RangeError are not intended to be catchable
raising and immediately catching an exception like this is slower and more verbose than an explicit test.

Rather than returning an empty array on an error and expecting a caller to catch RangeError, you could throw a normal exception on error, and then you have tools like std.exception.ifThrown to make dealing with that exception nicer.

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