getopt_long
To accept GNU-style long options as well as single-character options,
use getopt_long instead of getopt. This function is
declared in `getopt.h', not `unistd.h'. You should make every
program accept long options if it uses any options, for this takes
little extra work and helps beginners remember how to use the program.
getopt_long. The argument longopts must be an array of
these structures, one for each long option. Terminate the array with an
element containing all zeros.
The struct option structure has these fields:
const char *name
int has_arg
no_argument,
required_argument and optional_argument.
int *flag
int val
flag is a null pointer, then the val is a value which
identifies this option. Often these values are chosen to uniquely
identify particular long options.
If flag is not a null pointer, it should be the address of an
int variable which is the flag for this option. The value in
val is the value to store in the flag to indicate that the option
was seen.
getopt. The argument longopts describes the long
options to accept (see above).
When getopt_long encounters a short option, it does the same
thing that getopt would do: it returns the character code for the
option, and stores the options argument (if it has one) in optarg.
When getopt_long encounters a long option, it takes actions based
on the flag and val fields of the definition of that
option.
If flag is a null pointer, then getopt_long returns the
contents of val to indicate which option it found. You should
arrange distinct values in the val field for options with
different meanings, so you can decode these values after
getopt_long returns. If the long option is equivalent to a short
option, you can use the short option's character code in val.
If flag is not a null pointer, that means this option should just
set a flag in the program. The flag is a variable of type int
that you define. Put the address of the flag in the flag field.
Put in the val field the value you would like this option to
store in the flag. In this case, getopt_long returns 0.
For any long option, getopt_long tells you the index in the array
longopts of the options definition, by storing it into
*indexptr. You can get the name of the option with
longopts[*indexptr].name. So you can distinguish among
long options either by the values in their val fields or by their
indices. You can also distinguish in this way among long options that
set flags.
When a long option has an argument, getopt_long puts the argument
value in the variable optarg before returning. When the option
has no argument, the value in optarg is a null pointer. This is
how you can tell whether an optional argument was supplied.
When getopt_long has no more options to handle, it returns
-1, and leaves in the variable optind the index in
argv of the next remaining argument.
Since long option names were used before before the getopt_long
options was invented there are program interfaces which require programs
to recognize options like `-option value' instead of
`--option value'. To enable these programs to use the GNU
getopt functionality there is one more function available.
The getopt_long_only function is equivalent to the
getopt_long function but it allows to specify the user of the
application to pass long options with only `-' instead of
`--'. The `--' prefix is still recognized but instead of
looking through the short options if a `-' is seen it is first
tried whether this parameter names a long option. If not, it is parsed
as a short option.
Assuming getopt_long_only is used starting an application with
app -foo
the getopt_long_only will first look for a long option named
`foo'. If this is not found, the short options `f', `o',
and again `o' are recognized.
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