__int64 WINAPI atoi64(
  const char *Str
);

If the function succeeds, return value is the converted value of the input string, otherwise it returns 0i64.

The Str string parameter must be in the following form:
[ws][sn][ddd] where
  ws - space or tab characters (ignored)
  sn - sign - '+' or '-'
  ddd - one or more decimal digits - from '0' to '9'
The function stops reading the input string at the first character that it cannot recognize as part of a number. In case of overflow the return value is undefined.

You can define and initialize a function pointer to use it later:

FARSTDATOI64 FarAtoi64;
...
FarAtoi64=Info.FSF->atoi64;
...
I_64=FarAtoi64(Str);

...or call the function directly:

I_64=Info.FSF->atoi64(Str);