how do i convert std::vector to char ** in a reliable way?

Question

I have been trying to avoid using char ** arrays, however because i am using execvp, i wrote a method to convert a vector of strings to a null terminated char **.

Unfortunately i have until recently been getting segmentation faults, which makes me suspicious that my method doesn't always work for one reason or another:

void vectorToNullArray(std::vector<std::string> const &v, char **a) {
  int i = 0;
  while (i < v.size()) {
    std::string str = v[i];
    char *cstr = new char[str.size()+1];
    strcpy(cstr, str.c_str());
    a[i] = cstr;                       // <-- segfault here
    i++;
  }
  a[i] = nullptr;
}

if i can be completely free of char ** in the first place, that would be ideal. Why is this code segfaulting?

EDIT: this is how i call it:

  char **argvA;
    vectorToNullArray(_ARGV, argvA);

where _ARGV is an std::vector<std::string> , a member variable of some class.

EDIT 2: working solution

std::vector<char *> nullTerminatedArgV(std::vector<std::string> const &v) {
  std::vector<char *> result;
  for (auto const &s : v) {
    result.push_back(const_cast<char *>(s.c_str()));
  }
  result.push_back(nullptr);
  return result;
}

usage with execvp:

    std::vector<char *> argv = nullTerminatedArgV(_ARGV);
    char **command = argv.data();

      int status = execvp(command[0], command);
      perror("execvp error");

essentially the accepted answer, with additions from here:

https://stackoverflow.com/a/47714627/5133238

Answer 1

As Pete Becker says, the issue's probably that you haven't called it properly - and specifically that the a argument doesn't point to a sufficiently large array of char* s to write to.

Anyway, do you really need to screw around with dynamic memory allocation here? Having std::vector manage memory for you is much easier, safer, and more concise. Perhaps you can do something like below, then pass the result's .data() value to execvp ? You do need to make sure the vector of strings passed as an argument is not mutated by the caller after this call and before calling execvp , otherwise the pointers you collect may be invalidated.

std::vector<const char*> getVectorOfPointersToStrings(std::vector<std::string> const& v) {
    std::vector<const char*> result;
    for (const auto& s : v)
        result.push_back(s.c_str());
    result.push_back(nullptr);
    return result; 
}

Answer 2

char **argvA;
vectorToNullArray(_ARGV, argvA);

argvA has some random value; passing it to vectorToNullArray results in undefined behavior.

The code doesn't allocate the array itself, but assigns values to it.