The optionp parameter must be a pointer to a variable containing
the address of the string to process. When the function returns the
reference is updated to point to the next suboption or to the
terminating `\0' character if there is no more suboption available.
The tokens parameter references an array of strings containing the
known suboptions. All strings must be `\0' terminated and to mark
the end a null pointer must be stored. When
getsubopt finds a
possible legal suboption it compares it with all strings available in
the tokens array and returns the index in the string as the
In case the suboption has an associated value introduced by a `='
character, a pointer to the value is returned in valuep. The
string is `\0' terminated. If no argument is available
valuep is set to the null pointer. By doing this the caller can
check whether a necessary value is given or whether no unexpected value
In case the next suboption in the string is not mentioned in the
tokens array the starting address of the suboption including a
possible value is returned in valuep and the return value of the
function is `-1'.