Flags, i.e. letters in arguments/options that activate or deactivate certain switches can be checked using the function:
string unidentified_flags(const char* Known,
int ArgumentNumber /* =-1 */) const;
This function operates in two modes. In first mode, if the second argument is
omitted or set to -1, then all options starting with a single '-' are
considered and checked if there is any letter in them that is not in Known. Known is simply a string concatenating all possible flags
(such as 'xcvfjt' for tar). In the second mode, the argument number ArgumentNumber on the command line is checked for flags (argument 1 for
example in tar). The flags that are not contained in Known are listed
in the returned string.