Configuration and command line parser
=====================================

Libucw contains a parser for configuration files. The syntax of the
configuration files is described in <<config:>>, here we explain the
interface of the parser.

Basically, you write a description of the configuration file syntax,
which maps configuration items to variables of your program. Then
Then you run the parser and it fills your variables with the values
from the configuration file.

The descriptions are modular. The configuration can be split to sections,
each section declared at a separate place. You can also define your own
data types.

There is also a simple wrapper around getopt_long(), which processes
options related to selection of a configuration file, overriding of
configuration variables and loading of the default configuration.

- <<example,Example>>
  * <<ex_structure,The structure>>
  * <<ex_load,Loading>>
- <<deep,Getting deeper>>
  * <<conf_multi,Arrays and lists>>
  * <<reload,Reloading configuration>>
  * <<custom_parser,Creating custom parsers>>
  * <<hooks,Hooks>>
- <<conf_h,ucw/conf.h>>
  * <<conf_ctxt,Configuration contexts>>
  * <<conf_load,Safe configuration loading>>
  * <<conf_types,Data types>>
  * <<conf_macros,Convenience macros>>
  * <<alloc,Memory allocation>>
  * <<journal,Undo journal>>
  * <<declare,Section declaration>>
  * <<bparser,Parsers for basic types>>
  * <<conf_direct,Direct access>>
  * <<conf_dump,Debug dumping>>
- <<getopt_h,ucw/getopt.h>>
  * <<conf_getopt,Loading by cf_getopt()>>

[[example]]
Example
-------
If you want to just load simple configuration, this is the part you
want to read. This simple example should give you the overview. Look
at the <<conf_macros,convenience macros>> section to see list of
supported data types, sections, etc.

[[ex_cfile]]
Suppose you have configuration file with the following content and you
want to load it:

  HelloWorld {
    Text	"Hello planet"
    Count	3
  }

[[ex_structure]]
The structure
~~~~~~~~~~~~~
First, you declare the structure and let the configuration parser know
it exists.

  #include <ucw/lib.h>
  #include <ucw/conf.h>

  static char *hw_text = "Hello world";
  static int hw_count = 1;
  static int hw_wait_answer = 0;

  static struct cf_section hw_config = {
    CF_ITEMS {
      CF_STRING("Text", &hw_text),
      CF_INT("Count", &hw_count),
      CF_INT("WaitAnswer", &hw_wait_answer),
      CF_END
    }
  };

  static void CONSTRUCTOR hw_init(void) {
    cf_declare_section("HelloWorld", &hw_config, 0);
  }

The variables are used to store the loaded values. Their initial
values work as defaults, if nothing else is loaded. The hw_config()
structure assigns the variables to configuration names. The hw_init()
function (because of the `CONSTRUCTOR` macro) is run before main()
is called and it tells the parser that the section exists (alternatively,
you can call @cf_declare_section() at the start of your main()).

You can plug in as many configuration sections as you like, from
various places across your code.

[[ex_load]]
Loading of the values
~~~~~~~~~~~~~~~~~~~~~
Suppose you need to parse the command line arguments and load the
configuration. Then @cf_getopt() is there for you: it works like
the traditional @getopt_long() from the C library, but it also handles
configuration files.

  #include <ucw/lib.h>
  #include <ucw/conf.h>
  #include <ucw/getopt.h>

  static char short_opts[] = CF_SHORT_OPTS "v";
  static struct option long_opts[] = {
    CF_LONG_OPTS
    { "verbose", 0, 0, 'v' },
    { NULL, 0, 0, 0 }
  };

  static int verbose;

  int main(int argc, char *argv[]) {
    cf_def_file = "default.cf";
    int opt;
    while((opt = cf_getopt(argc, argv, short_opts, long_opts, NULL)) >= 0)
      switch(opt) {
	case 'v': verbose = 1; break;
	default: fprintf("Unknown option %c\n", opt); return 1;
      }
  }

The `short_opts` and `long_opts` variables describe the command line
arguments. Notice the `CF_SHORT_OPTS` and `CF_LONG_OPTS` macros. They
add the `-S` and `-C` options for the configuration parser as described
in <<config:>>. These options are handled internally by @cf_getopt().

You can rely on the configuration files having been loaded before the
first of your program's options is parsed.

[[deep]]
Getting deeper
--------------

Since the configuration system is somehow complicated, this part gives
you a little overview of what you can find and where.

[[conf_multi]]
Arrays and lists
~~~~~~~~~~~~~~~~

It is sometime needed to have multiple items of the same type. There
are three ways to do that:

*Static arrays*::
  An array with fixed maximum length. You provide
  the length and already allocated array which is filled with items.
  The configuration may contain less than the maximum length items.
+
For example, you can have an static array of five unsigned integers:
+
  static uns array[] = { 1, 2, 3, 4, 5 };
+
  static struct cf_section section = {
    CF_ITEMS {
      CF_UNS_ARY("array", array, 5),
      CF_END
    }
  };

*Dynamic arrays*::
  Similar to static array, but you provide pointer
  to pointer to the given item (eg. if you want dynamic array of
  integers, you give `**int`). The parser allocates an array of needed
  size. You can use the <<def_DARY_LEN,`DARY_LEN`>> macro to find out
  the number of elements actually loaded.
+
If you want dynamic array of strings, you would use:
+
  static char *array[];
+
  static struct cf_section section = {
    CF_ITEMS {
      CF_STRING_DYN("array", &array, CF_ANY_NUM),
      CF_END
    }
  };

*Lists*::
  Linked lists based on <<clist:>>. You provide description
  of single node and pointer to the
  <<clist:struct_clist,`struct clist`>> variable. All the nodes will
  be created dynamically and put there.
+
First element of your structure must be <<clist:type_cnode,`cnode`>>.
+
The first example is list of strings and uses <<clist:simple,simple
lists>>:
+
  static struct clist list;
+
  static struct cf_section section = {
    CF_ITEMS {
      CF_LIST("list", &list, &cf_string_list_config),
      CF_END
    }
  };
+
Another example, describing how to create more complicated list node
than just a string can be found at the <<def_CF_TYPE,`CF_TYPE`>> macro.

[[reload]]
Reloading configuration
~~~~~~~~~~~~~~~~~~~~~~~

The configuration system allows you to reload configuration at
runtime. The new config changes the values against the default values.
It means, if the default value for variable `A` is `10`, the currently
loaded config sets it to `42` and the new config does not talk about
this variable, `A` will have a value of `10` after a successful load.

Furthermore, if the loading of a new configuration fails, the current
configuration is preserved.

All this is done with <<journal,config journalling>>. The load of the
first config creates a journal entry. If you try to load some new
configuration, it is partially rolled back to defaults (the rollback
happens, but instead of removing the journal entry, another journal
entry is added for the rollback). If the loading succeeds, the two
journal entries are removed and a new one, for the new configuration,
is added. If it fails, the first one is replayed and the rollback
entry is removed.

See <<cf_reload()>>.

[[custom_parser]]
Creating custom parsers
~~~~~~~~~~~~~~~~~~~~~~~

If you need to parse some data type the configuration system can't
handle, you can write your own parser. But before you start, you
should know a few things.

The parser needs to support <<journal,journalling>>. To accomplish that,
you have to use the <<alloc,configuration mempool>> for memory allocation.

Now, you need a function with the same signature as
<<type_cf_parser1,`cf_parser1`>>. Parse the first parameter (the
string) and store the data in the second parameter. You may want to
write a dumper function, with signature of
<<type_cf_dumper1,`cf_dumper1`>> (needed for debug dumps).

Fill in a structure <<struct_cf_user_type,cf_user_type>> and use the
new data type in your configuration description with
<<def_CF_USER,`CF_USER`>> macro as its @t parameter.

You do not need to call @cf_journal_block() on the variable you store
the result. It is true you change it, but it was stored to journal
before your parser function was called.

[[hooks]]
Hooks
~~~~~

The configuration system supports hooks. They are used to initialize the
configuration (if simple default value of variable is not enough) and
to check the sanity of loaded data.

Each hook is of type <<type_cf_hook,`cf_hook`>> and you can include
them in configuration description using <<def_CF_INIT,`CF_INIT`>> and
<<def_CF_COMMIT,`CF_COMMIT`>> macros.

The hooks should follow similar guidelines as custom parsers (well,
init hooks do not need to call @cf_journal_block()) to support
journalling. If you change nothing in the commit hook, you do not need
to care about the journalling either.

You may use the return value to inform about errors. Just return the
error message, or NULL if everything went well.

Another similar function is a copy function. It is very similar to a
hook and is used when the item is copied and is too complicated to use
simple memcpy(). Its type is <<type_cf_copier,`cf_copier`>> and is
specified by the <<def_CF_COPY,`CF_COPY`>> macro. It's return value is
the same as the one of a hook.

[[conf_h]]
ucw/conf.h
----------

This header file contains the public interface of the configuration module.

!!ucw/conf.h

[[getopt_h]]
ucw/getopt.h
------------

This header contains routines for parsing command line arguments and
loading the default configuration.

!!ucw/getopt.h