parse-options API
=================

The parse-options API is used to parse and massage options in Git
and to provide a usage help with consistent look.

Basics
------

The argument vector `argv[]` may usually contain mandatory or optional
'non-option arguments', e.g. a filename or a branch, and 'options'.
Options are optional arguments that start with a dash and
that allow to change the behavior of a command.

* There are basically three types of options:
  'boolean' options,
  options with (mandatory) 'arguments' and
  options with 'optional arguments'
  (i.e. a boolean option that can be adjusted).

* There are basically two forms of options:
  'Short options' consist of one dash (`-`) and one alphanumeric
  character.
  'Long options' begin with two dashes (`--`) and some
  alphanumeric characters.

* Options are case-sensitive.
  Please define 'lower-case long options' only.

The parse-options API allows:

* 'stuck' and 'separate form' of options with arguments.
  `-oArg` is stuck, `-o Arg` is separate form.
  `--option=Arg` is stuck, `--option Arg` is separate form.

* Long options may be 'abbreviated', as long as the abbreviation
  is unambiguous.

* Short options may be bundled, e.g. `-a -b` can be specified as `-ab`.

* Boolean long options can be 'negated' (or 'unset') by prepending
  `no-`, e.g. `--no-abbrev` instead of `--abbrev`. Conversely,
  options that begin with `no-` can be 'negated' by removing it.
  Other long options can be unset (e.g., set string to NULL, set
  integer to 0) by prepending `no-`.

* Options and non-option arguments can clearly be separated using the `--`
  option, e.g. `-a -b --option -- --this-is-a-file` indicates that
  `--this-is-a-file` must not be processed as an option.

Steps to parse options
----------------------

. `#include "parse-options.h"`

. define a NULL-terminated
  `static const char * const builtin_foo_usage[]` array
  containing alternative usage strings

. define `builtin_foo_options` array as described below
  in section 'Data Structure'.

. in `cmd_foo(int argc, const char **argv, const char *prefix)`
  call

        argc = parse_options(argc, argv, prefix, builtin_foo_options, builtin_foo_usage, flags);
+
`parse_options()` will filter out the processed options of `argv[]` and leave the
non-option arguments in `argv[]`.
`argc` is updated appropriately because of the assignment.
+
You can also pass NULL instead of a usage array as the fifth 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`::
        Keep the `--` that usually separates options from
        non-option arguments.

`PARSE_OPT_STOP_AT_NON_OPTION`::
        Usually the whole argument vector is massaged and reordered.
        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
--------------

The main data structure is an array of the `option` struct,
say `static struct option builtin_add_options[]`.
There are some macros to easily define options:

`OPT__ABBREV(&int_var)`::
        Add `--abbrev[=<n>]`.

`OPT__COLOR(&int_var, description)`::
        Add `--color[=<when>]` and `--no-color`.

`OPT__DRY_RUN(&int_var, description)`::
        Add `-n, --dry-run`.

`OPT__FORCE(&int_var, description)`::
        Add `-f, --force`.

`OPT__QUIET(&int_var, description)`::
        Add `-q, --quiet`.

`OPT__VERBOSE(&int_var, description)`::
        Add `-v, --verbose`.

`OPT_GROUP(description)`::
        Start an option group. `description` is a short string that
        describes the group or an empty string.
        Start the description with an upper-case letter.

`OPT_BOOL(short, long, &int_var, description)`::
        Introduce a boolean option. `int_var` is set to one with
        `--option` and set to zero with `--no-option`.

`OPT_COUNTUP(short, long, &int_var, description)`::
        Introduce a count-up option.
        `int_var` is incremented on each use of `--option`, and
        reset to zero with `--no-option`.

`OPT_BIT(short, long, &int_var, description, mask)`::
        Introduce a boolean option.
        If used, `int_var` is bitwise-ored with `mask`.

`OPT_NEGBIT(short, long, &int_var, description, mask)`::
        Introduce a boolean option.
        If used, `int_var` is bitwise-anded with the inverted `mask`.

`OPT_SET_INT(short, long, &int_var, description, integer)`::
        Introduce an integer option.
        `int_var` is set to `integer` with `--option`, and
        reset to zero with `--no-option`.

`OPT_STRING(short, long, &str_var, arg_str, description)`::
        Introduce an option with string argument.
        The string argument is put into `str_var`.

`OPT_INTEGER(short, long, &int_var, description)`::
        Introduce an option with integer argument.
        The integer is put into `int_var`.

`OPT_DATE(short, long, &int_var, description)`::
        Introduce an option with date argument, see `approxidate()`.
        The timestamp is put into `int_var`.

`OPT_EXPIRY_DATE(short, long, &int_var, description)`::
        Introduce an option with expiry date argument, see `parse_expiry_date()`.
        The timestamp is put into `int_var`.

`OPT_CALLBACK(short, long, &var, arg_str, description, func_ptr)`::
        Introduce an option with argument.
        The argument will be fed into the function given by `func_ptr`
        and the result will be put into `var`.
        See 'Option Callbacks' below for a more elaborate description.

`OPT_FILENAME(short, long, &var, description)`::
        Introduce an option with a filename argument.
        The filename will be prefixed by passing the filename along with
        the prefix argument of `parse_options()` to `prefix_filename()`.

`OPT_ARGUMENT(long, description)`::
        Introduce a long-option argument that will be kept in `argv[]`.

`OPT_NUMBER_CALLBACK(&var, description, func_ptr)`::
        Recognize numerical options like -123 and feed the integer as
        if it was an argument to the function given by `func_ptr`.
        The result will be put into `var`.  There can be only one such
        option definition.  It cannot be negated and it takes no
        arguments.  Short options that happen to be digits take
        precedence over it.

`OPT_COLOR_FLAG(short, long, &int_var, description)`::
        Introduce an option that takes an optional argument that can
        have one of three values: "always", "never", or "auto".  If the
        argument is not given, it defaults to "always".  The `--no-` form
        works like `--long=never`; it cannot take an argument.  If
        "always", set `int_var` to 1; if "never", set `int_var` to 0; if
        "auto", set `int_var` to 1 if stdout is a tty or a pager,
        0 otherwise.

`OPT_NOOP_NOARG(short, long)`::
        Introduce an option that has no effect and takes no arguments.
        Use it to hide deprecated options that are still to be recognized
        and ignored silently.

`OPT_PASSTHRU(short, long, &char_var, arg_str, description, flags)`::
        Introduce an option that will be reconstructed into a char* string,
        which must be initialized to NULL. This is useful when you need to
        pass the command-line option to another command. Any previous value
        will be overwritten, so this should only be used for options where
        the last one specified on the command line wins.


The last element of the array must be `OPT_END()`.

If not stated otherwise, interpret the arguments as follows:

* `short` is a character for the short option
  (e.g. `'e'` for `-e`, use `0` to omit),

* `long` is a string for the long option
  (e.g. `"example"` for `--example`, use `NULL` to omit),

* `int_var` is an integer variable,

* `str_var` is a string variable (`char *`),

* `arg_str` is the string that is shown as argument
  (e.g. `"branch"` will result in `<branch>`).
  If set to `NULL`, three dots (`...`) will be displayed.

* `description` is a short string to describe the effect of the option.
  It shall begin with a lower-case letter and a full stop (`.`) shall be
  omitted at the end.

Option Callbacks
----------------

The function must be defined in this form:

        int func(const struct option *opt, const char *arg, int unset)

The callback mechanism is as follows:

* Inside `func`, the only interesting member of the structure
  given by `opt` is the void pointer `opt->value`.
  `*opt->value` will be the value that is saved into `var`, if you
  use `OPT_CALLBACK()`.
  For example, do `*(unsigned long *)opt->value = 42;` to get 42
  into an `unsigned long` variable.

* Return value `0` indicates success and non-zero return
  value will invoke `usage_with_options()` and, thus, die.

* If the user negates the option, `arg` is `NULL` and `unset` is 1.

Sophisticated option parsing
----------------------------

If you need, for example, option callbacks with optional arguments
or without arguments at all, or if you need other special cases,
that are not handled by the macros above, you need to specify the
members of the `option` structure manually.

This is not covered in this document, but well documented
in `parse-options.h` itself.

Examples
--------

See `test-parse-options.c` and
`builtin/add.c`,
`builtin/clone.c`,
`builtin/commit.c`,
`builtin/fetch.c`,
`builtin/fsck.c`,
`builtin/rm.c`
for real-world examples.