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

Question

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.

Please see this discussion on why not to cast the return value of malloc() and family in C.. — Sourav Ghosh
@MikeMB Isn't it like we have C compilers to compile C code? We should be using that, anyway. BTW, that was just an informative comment. — Sourav Ghosh
@Ajay C++ has new, better use that. You can shoot your foot, but that does not mean you have to. :) — Sourav Ghosh

Ajay Ajay · Accepted Answer · 2016-05-13T08:48:03

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

Disable the given warning around the code using #pragma directive.
Use C++ constructs: new, make_unique etc.
(Not recommended) is to ignore the 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 the call stack. You don't need to type * at all!

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

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

2 Answers