UniStrtod converts the initial portion of the string pointed to by nptr to double-precision floating-point representation. First, it decomposes the input string into three parts:

• An initial, possibly empty, sequence of white-space characters (as indicated by the space attribute).

• A subject sequence resembling a floating-point constant.

• A final string of one or more unrecognized characters, including the ending null character of the input string.

Then, it attempts to convert the subject sequence to a floating-point number, and returns the result in the area pointed to by fResult. A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.