#include <stdlib.h>double atof(const char *str);
The atof( ) function converts the string pointed to by str into a double value. The string must contain a valid floating-point number. If this is not the case, the returned value is undefined.
The number can be terminated by any character that cannot be part of a valid floating-point number. This includes whitespace, punctuation (other than periods), and characters other than E or e. This means that if atof( ) is called with “100.00HELLO”, the value 100.00 will be returned.
Related functions are atoi( ) and atol( ).