I want to read a `long` number from file then increment it and write it back to file.   

I am struggling with the convertion from `string` to `long` and back again.

I tried:

```
double id = atof("12345678901"); //using atof because numbers are too big for atio()
id++;
ostringstream strs;
strs << static_cast<long>((static_cast<double>(threadId)));
string output = strcpy_s(config->m_threadId, 20, strs.str().c_str());

```

But that converts the input to a negative or wrong number.