CODEWITHSUNDEEP
c++debuggingmemoryundefined-behavior
bobbaluba
bobbaluba

Reputation: 3894

How can I find out what's changing the return address of a function in c++

I have a program that behaves weirdly and probably has undefined behaviour. Sometimes, the return address of a function seems to be changed, and I don't know what's causing it.

The return address is always changed to the same address, an assertion inside a function the control shouldn't be able to reach. I've been able to stop the program with a debugger to see that when it's supposed to execute a return statement, it jumps straight to the line with the assertion instead.

This code approximates how my function works.

int foo(Vector t)
   double sum = 0;
   for(unsgined int i=0; i<t.size();++i){
        sum += t[i]; 
   }
   double limit = bar(); // bar returns a value between 0 and 1
   double a=0;
   for(double i=0; i<10; i++){
       a += f(i)/sum; // f(1)/sum + ... + f(10)/sum = 1.0f
       if(a>3)return a;
   }
   //shoudn'get here
   assert(false); // ... then this line is executed
}

This is what I've tried so far:

I get a couple of invalid read of size 8, but they seem to originate from qt, so I'm not sure what to make of it. Could this be the problem?

The error happens only occasionally when I have run the program for a while and give it certain input values, and more often in a release build than in a debug build.

EDIT: So I managed to reproduce the problem in a console application (no qt loaded) I then manages to simulate events that caused the problem.

Like some of you suggested, it turns out I misjudged what was actually causing it to reach the assertion, probably due to my lack of experience with qt's debugger. The actual problem was a floating point error in the double i used as a loop condition.

I was implementing softmax, but exp(x) got rounded to zero with particular inputs.

Now, as I have solved the problem, I might rephrase it. Is there a method for checking problems like rounding errors automatically. I.e breaking on 0/0 for instance?

Upvotes: 2

Views: 217

Answers (1)

Stefano Falasca
Stefano Falasca

Reputation: 9097

The short answer is:

The most portable way of determining if a floating-point exceptional condition has occurred is to use the floating-point exception facilities provided by C in fenv.h.

although, unfortunately, this is far from being perfect.

I suggest you to read both https://www.securecoding.cert.org/confluence/display/seccode/FLP04-C.+Check+floating-point+inputs+for+exceptional+values and https://www.securecoding.cert.org/confluence/display/seccode/FLP03-C.+Detect+and+handle+floating-point+errors which concisely address the exact question you are posing:

Is there a method for checking problems like rounding errors automatically.

Upvotes: 1

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