Integer value corrupt when casting from void*

Question

I have the following code:

typedef void* EstimatedValue;

EstimatedValue myFunction()
{
    int myVal = 16;
    EstimatedValue value = &myVal ;
    return value;
}
bool intLessOrEqual(EstimatedValue value1, EstimatedValue value2)
{
    return *(int*)value1 <= *(int*)value2;
}

And this main function:

int x = 8;
EstimatedValue val = myFunction();
EstimatedValue x1 = &x;
intLessOrEqual(x1, val);

However, the value of the second argument (the one that is created in the function) is corrupt inside the intLessOrEqual function (the value is -858993460). If I try to access *(int*)val inside the main function it works ok. What could be the reason for this behavior? What can be done in order to avoid this behavior

`myVal` is declared local to the function `myfunction`. You are returning the address of a local variable. This is undefined behavior. — MFisherKDX, Sep 09 '18 at 16:30

P.W · Accepted Answer · 2018-09-09T16:35:19.310

1

myVal is created on the stack in the myFunction function. And you are returning an address to it. This leads to undefined behaviour because the value of myVal can change once it goes out of scope.

What you can do is create memory for value like this:

EstimatedValue value = malloc(sizeof(int);

Assign the value:

*value = 16;
return value;

In main after all the processing, you have to free that memory like this:

  free(val);

edited Sep 09 '18 at 16:35

answered Sep 09 '18 at 16:29

P.W

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What would you do in order to avoid this behavior? – Nave Tseva Sep 09 '18 at 16:34
Updated the answer to avoid the behaviour. – P.W Sep 09 '18 at 16:36

score 1 · Answer 2 · answered Sep 09 '18 at 16:29

1

The problem is here:

EstimatedValue myFunction()
{
    int myVal = 16;                  // local variable
    EstimatedValue value = &myVal ;
    return value;                    // now release where myVal was
}

By the time this function have returned back to its caller, the memory where the local variable myVal is saved will have been released, and val in EstimatedValue val = myFunction(); essentially is pointing a memory space that is not occupied, i.e. anything else can be overwritten on it. Then what happens next is unpredictable.

answered Sep 09 '18 at 16:29

fractals

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Thanks, so making EstimatedValue as one of the input parameters of the function will solve the problem? – Nave Tseva Sep 09 '18 at 16:32
Yes, you can essentially get a buffer passed as the argument of `myFunction()`. – fractals Sep 09 '18 at 16:35

Integer value corrupt when casting from void*

2 Answers2