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 25.3.12.1 Parsing of Suboptions
 ...............................
 
 Having a single level of options is sometimes not enough.  There might
 be too many options which have to be available or a set of options is
 closely related.
 
    For this case some programs use suboptions.  One of the most
 prominent programs is certainly 'mount'(8).  The '-o' option take one
 argument which itself is a comma separated list of options.  To ease the
 programming of code like this the function 'getsubopt' is available.
 
  -- Function: int getsubopt (char **OPTIONP, char *const *TOKENS, char
           **VALUEP)
      Preliminary: | MT-Safe | AS-Safe | AC-Safe |  POSIX Safety
      Concepts.
 
      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'.
 

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