non-option arguments in `argv[]`.
`argc` is updated appropriately because of the assignment.
+
+You can also pass NULL instead of a usage array as fourth parameter of
+parse_options(), to avoid displaying a help screen with usage info and
+option list. This should only be done if necessary, e.g. to implement
+a limited parser for only a subset of the options that needs to be run
+before the full parser, which in turn shows the full help message.
++
Flags are the bitwise-or of:
`PARSE_OPT_KEEP_DASHDASH`::
Using this flag, processing is stopped at the first non-option
argument.
+`PARSE_OPT_KEEP_ARGV0`::
+ Keep the first argument, which contains the program name. It's
+ removed from argv[] by default.
+
+`PARSE_OPT_KEEP_UNKNOWN`::
+ Keep unknown arguments instead of erroring out. This doesn't
+ work for all combinations of arguments as users might expect
+ it to do. E.g. if the first argument in `--unknown --known`
+ takes a value (which we can't know), the second one is
+ mistakenly interpreted as a known option. Similarly, if
+ `PARSE_OPT_STOP_AT_NON_OPTION` is set, the second argument in
+ `--unknown value` will be mistakenly interpreted as a
+ non-option, not as a value belonging to the unknown option,
+ the parser early. That's why parse_options() errors out if
+ both options are set.
+
+`PARSE_OPT_NO_INTERNAL_HELP`::
+ By default, parse_options() handles `-h`, `--help` and
+ `--help-all` internally, by showing a help screen. This option
+ turns it off and allows one to add custom handlers for these
+ options, or to just leave them unknown.
+
Data Structure
--------------