Сиротин - Лабораторная работа #3 

http://www.cplusplus.com/reference/cstdlib/strtod/
On success, the function returns the converted floating point number as a value of type double.
If no valid conversion could be performed, the function returns zero (0.0).
If the correct value is out of the range of representable values for the type, a positive or negative HUGE_VAL is returned, and errno is set to ERANGE.