Question

I’m trying to debug a legacy code written for Linux. Sometimes the application gets a segfault when it reaches the memcpy call in the following method:

std::vector<uint8> _storage;
size_t _wpos;
void append(const uint8 *src, size_t cnt)
{
  if (!cnt)
  return;

  if (_storage.size() < _wpos + cnt)
    _storage.resize(_wpos + cnt);
  memcpy(&_storage[_wpos], src, cnt);
  _wpos += cnt;
}

The values are as follows:

_storage.size() is 1000
_wpos is 0
*src points to an array of uint8 with 3 values: { 3, 110, 20 }
cnt is 3

I have no idea why this happens since this method gets called thousands of times during the application’s runtime but it sometimes gets a segfault.

Any one has any idea how to solve this?

Answer 1

Your code looks good in terms of the data that is written. Are you absolutely sure that you’re passing in the right src pointer? What happens when you run the code with a debugger such as gdb? It should halt on the segfault, and then you can print out the values of _storage.size(), src, and cnt.

I’m sure you’ll find that (at least) one of those is not at all what you’re expecting. You might have passed an invalid src; you might have passed an absurdly large cnt.