VS2015: [C6386] Buffer Overrun while writing (even for same index value)

问题

I'm trying to implement merge sort in C when I came across something interesting raised by [Analyze -> Run Code Analysis] in Visual Studio 2015.

The code is as follows:

void MergeSort_r(int A[], int n)
{
    // A = {1, 3, 2}
    // n = 3
    int rightCount;
    int* R;

    if ( n < 2 ) return;

    // version 1: rightCount = 2
    rightCount = n - (n/2);

    // version 2: rightCount = 2
    rightCount = n - 1;

    R = ( int* ) malloc( rightCount * sizeof( int ) );

    if ( R ) {
        for ( int i = 0; i < rightCount; i++ ) {
            R[i] = A[i];
        }

    free( R );
    }

}

Even though both version of rightCount essentially evaluates to 2, in the first version, I get the warning:

"Buffer overrun while writing to 'R': the writable size is '(unsigned int)rightCount*sizeof(int)' bytes, but '8' bytes might be written."

Any idea why this is the case? Looking forward to hear your answers.

回答1:

Visual C++ Code Analysis tool set may not always offer best warnings. It tries to give you best set of warnings to fix some potential issues/errors that may creep in at runtime. You have few options:

Disable the given warning around the code using #pragma directive.
Use C++ constructs: new, make_unique etc.
(Not recommended) is to ignore warning altogether and move on.

You should ideally always user newer smart pointers primitives like unique_ptr, shared_ptr etc. They not only allocate memory for you, but deallocate on any exception thrown across call stack. You don't need to type * totally!

auto buffer = make_unique<int[]>(10); // 10 integers

回答2:

Your code is fine and tools(especially analyzers) have their drawbacks — sometimes they generate false-positives. That's one of it. BTW, I checked your code on MSVS2015 and it gives me no warnings.

来源：https://stackoverflow.com/questions/37205179/vs2015-c6386-buffer-overrun-while-writing-even-for-same-index-value

标签

c++

visual-studio-2015