Unusual C construct - What is it?

From:

Ramon F Herrera <ramon@conexus.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 23 Dec 2007 22:07:50 -0800 (PST)

Message-ID:

<a052bc0d-3eca-4bff-8dbe-dec31062b68a@n20g2000hsh.googlegroups.com>

I was looking at some open source code, and found something that I had
never seen before:

const struct ast_datastore_info dialed_interface_info = {
    .type ="dialed-interface",
    .destroy = dialed_interface_destroy,
    .duplicate = dialed_interface_duplicate,
};

What the heck are those dotted variables (?) fields(?). I don't recall
seeing that in the K&R.

I include the full file below.

-Ramon

---------------
/*
* Asterisk -- An open source telephony toolkit.
*
* Copyright (C) 1999 - 2007, Digium, Inc.
*
* Mark Michelson <mmichelson@digium.com>
*
* See http://www.asterisk.org for more information about
* the Asterisk project. Please do not directly contact
* any of the maintainers of this project for assistance;
* the project provides a web site, mailing lists and IRC
* channels for your use.
*
* This program is free software, distributed under the terms of
* the GNU General Public License Version 2. See the LICENSE file
* at the top of the source tree.
*/

/*! \file
*
* \brief globally-accessible datastore information and callbacks
*
* \author Mark Michelson <mmichelson@digium.com>
*/

#include "asterisk/global_datastores.h"
#include "asterisk/linkedlists.h"

static void dialed_interface_destroy(void *data)
{
    struct ast_dialed_interface *di = NULL;
    AST_LIST_HEAD(, ast_dialed_interface) *dialed_interface_list = data;

    if (!dialed_interface_list)
        return;

    AST_LIST_LOCK(dialed_interface_list);
    while ((di = AST_LIST_REMOVE_HEAD(dialed_interface_list, list)))
        ast_free(di);
    AST_LIST_UNLOCK(dialed_interface_list);

    AST_LIST_HEAD_DESTROY(dialed_interface_list);
    ast_free(dialed_interface_list);
}

static void *dialed_interface_duplicate(void *data)
{
    struct ast_dialed_interface *di = NULL;
    AST_LIST_HEAD(, ast_dialed_interface) *old_list;
    AST_LIST_HEAD(, ast_dialed_interface) *new_list = NULL;

    if(!(old_list = data))
        return NULL;

    if(!(new_list = ast_calloc(1, sizeof(*new_list))))
        return NULL;

    AST_LIST_HEAD_INIT(new_list);
    AST_LIST_LOCK(old_list);
    AST_LIST_TRAVERSE(old_list, di, list) {
        struct ast_dialed_interface *di2 = ast_calloc(1, sizeof(*di2) +
strlen(di->interface));
        if(!di2) {
            AST_LIST_UNLOCK(old_list);
            dialed_interface_destroy(new_list);
            return NULL;
        }
        strcpy(di2->interface, di->interface);
        AST_LIST_INSERT_TAIL(new_list, di2, list);
    }
    AST_LIST_UNLOCK(old_list);

    return new_list;
}

const struct ast_datastore_info dialed_interface_info = {
    .type ="dialed-interface",
    .destroy = dialed_interface_destroy,
    .duplicate = dialed_interface_duplicate,
};