The strsep function locates in stringp, the first occurrence of
    any character in delim (or the terminating '\0' character) and
    replaces it with a '\0'. The location of the next character after
    the delimiter character (or NULL, if the end of the string is
    reached) is stored in the stringp argument. The original value of
    the stringp argument is returned.

    You can detect an "empty" field; one caused by two adjacent
    delimiter characters, by comparing the location referenced by
    the pointer returned in the stringp argument to '\0'.

    The stringp argument is initially NULL, strsep returns NULL.