technical/api-parse-options.txt

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:

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

* 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, 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.
+
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.

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__DRY_RUN(&int_var)`::
Add `-n, \--dry-run`.

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

`OPT__VERBOSE(&int_var)`::
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_BOOLEAN(short, long, &int_var, description)`::
Introduce a boolean option.
`int_var` is incremented on each use.

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

`OPT_SET_INT(short, long, &int_var, description, integer)`::
Introduce a boolean option.
If used, set `int_var` to `integer`.

`OPT_SET_PTR(short, long, &ptr_var, description, ptr)`::
Introduce a boolean option.
If used, set `ptr_var` to `ptr`.

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


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 `funct`, 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.