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 indicator.

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 is present.

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'.