config API
==========

The config API gives callers a way to access git configuration files
(and files which have the same syntax). See linkgit:git-config[1] for a
discussion of the config file syntax.

General Usage
-------------

Config files are parsed linearly, and each variable found is passed to a
caller-provided callback function. The callback function is responsible
for any actions to be taken on the config option, and is free to ignore
some options (it is not uncommon for the configuration to be parsed
several times during the run of a git program, with different callbacks
picking out different variables useful to themselves).

A config callback function takes three parameters:

- the name of the parsed variable. This is in canonical "flat" form: the
  section, subsection, and variable segments will be separated by dots,
  and the section and variable segments will be all lowercase. E.g.,
  `core.ignorecase`, `diff.SomeType.textconv`.

- the value of the found variable, as a string. If the variable had no
  value specified, the value will be NULL (typically this means it
  should be interpreted as boolean true).

- a void pointer passed in by the caller of the config API; this can
  contain callback-specific data

A config callback should return 0 for success, or -1 if the variable
could not be parsed properly.

Basic Config Querying
---------------------

Most programs will simply want to look up variables in all config files
that git knows about, using the normal precedence rules. To do this,
call `git_config` with a callback function and void data pointer.

`git_config` will read all config sources in order of increasing
priority. Thus a callback should typically overwrite previously-seen
entries with new ones (e.g., if both the user-wide `~/.gitconfig` and
repo-specific `.git/config` contain `color.ui`, the config machinery
will first feed the user-wide one to the callback, and then the
repo-specific one; by overwriting, the higher-priority repo-specific
value is left at the end).

There is a special version of `git_config` called `git_config_early`
that takes an additional parameter to specify the repository config.
This should be used early in a git program when the repository location
has not yet been determined (and calling the usual lazy-evaluation
lookup rules would yield an incorrect location).

Reading Specific Files
----------------------

To read a specific file in git-config format, use
`git_config_from_file`. This takes the same callback and data parameters
as `git_config`.

Value Parsing Helpers
---------------------

To aid in parsing string values, the config API provides callbacks with
a number of helper functions, including:

`git_config_int`::
Parse the string to an integer, including unit factors. Dies on error;
otherwise, returns the parsed result.

`git_config_ulong`::
Identical to `git_config_int`, but for unsigned longs.

`git_config_bool`::
Parse a string into a boolean value, respecting keywords like "true" and
"false". Integer values are converted into true/false values (when they
are non-zero or zero, respectively). Other values cause a die(). If
parsing is successful, the return value is the result.

`git_config_bool_or_int`::
Same as `git_config_bool`, except that integers are returned as-is, and
an `is_bool` flag is unset.

`git_config_maybe_bool`::
Same as `git_config_bool`, except that it returns -1 on error rather
than dying.

`git_config_string`::
Allocates and copies the value string into the `dest` parameter; if no
string is given, prints an error message and returns -1.

`git_config_pathname`::
Similar to `git_config_string`, but expands `~` or `~user` into the
user's home directory when found at the beginning of the path.

Writing Config Files
--------------------

TODO