Question

Consider the following code.

#include <stdio.h>
#include <vector>
#include <iostream>

struct XYZ { int X,Y,Z; };
std::vector<XYZ> A;

int rec(int idx)
{

   int i = A.size();
   A.push_back(XYZ());
   if (idx >= 5)
     return i;

   A[i].X = rec(idx+1);

   return i;
}

int main(){
  A.clear();
  rec(0);
  puts("FINISH!");

}

I couldn’t figure out the reason why the code gives a segmentation fault on Linux (IDE used: Code::Blocks) whereas on Windows (IDE used: Visual C++) it doesn’t.

When I used Valgrind just to check what actually the problem was, I got this output.

I got Invalid write of size 4 at four different places. Then why didn’t the code crash when I used Visual C++?

Am I missing something?

Answer 1

The recursive call to rec() might modify the vector while you’re assigning a value to it.

What happens if you replace

A[i].X = rec(idx+1);

with

int tmp = rec(idx+1);
A[i].X = tmp;

?

Also, just to summarize the useful comments: the operand evaluation order of a = operation is unspecified and since the vector wasn’t preallocated, several resizes can occur during a recursive call to rec(), thus invalidating any iterator to values in the vector.