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, 'options', and
'subcommands'.
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.

Subcommands are special in a couple of ways:

* Subcommands only have long form, and they have no double dash prefix, no
  negated form, and no description, and they don't take any arguments, and
  can't be abbreviated.

* There must be exactly one subcommand among the arguments, or zero if the
  command has a default operation mode.

* All arguments following the subcommand are considered to be arguments of
  the subcommand, and, conversely, arguments meant for the subcommand may
  not precede the subcommand.

Therefore, if the options array contains at least one subcommand and
`parse_options()` encounters the first dashless argument, it will either:

* stop and return, if that dashless argument is a known subcommand, setting
  `value` to the function pointer associated with that subcommand, storing
  the name of the subcommand in argv[0], and leaving the rest of the
  arguments unprocessed, or

* stop and return, if it was invoked with the `PARSE_OPT_SUBCOMMAND_OPTIONAL`
  flag and that dashless argument doesn't match any subcommands, leaving
  `value` unchanged and the rest of the arguments unprocessed, or

* show error and usage, and abort.

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_OPT`::
	Keep unknown options 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.
	Note that non-option arguments are always kept, even without
	this flag.

`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.

`PARSE_OPT_SUBCOMMAND_OPTIONAL`::
	Don't error out when no subcommand is specified.

Note that `PARSE_OPT_STOP_AT_NON_OPTION` is incompatible with subcommands;
while `PARSE_OPT_KEEP_DASHDASH` and `PARSE_OPT_KEEP_UNKNOWN_OPT` can only be
used with subcommands when combined with `PARSE_OPT_SUBCOMMAND_OPTIONAL`.

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.
	Each use of `--option` increments `int_var`, starting from zero
	(even if initially negative), and `--no-option` resets it to
	zero. To determine if `--option` or `--no-option` was encountered at
	all, initialize `int_var` to a negative value, and if it is still
	negative after parse_options(), then neither `--option` nor
	`--no-option` was seen.

`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_STRING_LIST(short, long, &struct string_list, arg_str, description)`::
	Introduce an option with string argument.
	The string argument is stored as an element in `string_list`.
	Use of `--no-option` will clear the list of preceding values.

`OPT_INTEGER(short, long, &int_var, description)`::
	Introduce an option with integer argument. The argument must be a
	integer and may include a suffix of 'k', 'm' or 'g' to
	scale the provided value by 1024, 1024^2 or 1024^3 respectively.
	The scaled value is put into `int_var`.

`OPT_UNSIGNED(short, long, &unsigned_long_var, description)`::
	Introduce an option with an unsigned integer argument. The argument must be a
	non-negative integer and may include a suffix of 'k', 'm' or 'g' to
	scale the provided value by 1024, 1024^2 or 1024^3 respectively.
	The scaled value is put into `unsigned_long_var`.

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

`OPT_PASSTHRU_ARGV(short, long, &strvec_var, arg_str, description, flags)`::
	Introduce an option where all instances of it on the command-line will
	be reconstructed into a strvec. This is useful when you need to
	pass the command-line option, which can be specified multiple times,
	to another command.

`OPT_CMDMODE(short, long, &int_var, description, enum_val)`::
	Define an "operation mode" option, only one of which in the same
	group of "operating mode" options that share the same `int_var`
	can be given by the user. `int_var` is set to `enum_val` when the
	option is used, but an error is reported if other "operating mode"
	option has already set its value to the same `int_var`.
	In new commands consider using subcommands instead.

`OPT_SUBCOMMAND(long, &fn_ptr, subcommand_fn)`::
	Define a subcommand.  `subcommand_fn` is put into `fn_ptr` when
	this subcommand is used.

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.
