/*
* Set up default settings. If the configuration file exist, the values
* there will have precedence. Otherwise, the server address is set to
* INADDR_ANY and the default domain name comes from the gethostname().
*
* An interrim version of this code (BIND 4.9, pre-4.4BSD) used 127.0.0.1
* rather than INADDR_ANY ("0.0.0.0") as the default name server address
* since it was noted that INADDR_ANY actually meant ``the first interface
* you "ifconfig"'d at boot time'' and if this was a SLIP or PPP interface,
* it had to be "up" in order for you to reach your own name server. It
* was later decided that since the recommended practice is to always
* install local static routes through 127.0.0.1 for all your network
* interfaces, that we could solve this problem without a code change.
*
* The configuration file should always be used, since it is the only way
* to specify a default domain. If you are running a server on your local
* machine, you should say "nameserver 0.0.0.0" or "nameserver 127.0.0.1"
* in the configuration file.
*
* Return 0 if completes successfully, -1 on error
*/
安裝默認設置。如果配置文件存在，這些值將會有優先級順序。否則，服務器地址將被設置成INADDR_ANY（0.0.0.0）並且默認域名來自gethostname().這些代碼的一個中間的過渡版本(BIND 4.9, pre-4.4BSD)使用127.0.0.1而不是INADDR_ANY（0.0.0.0）作為默認的域名服務器地址因為很顯然INADDR_ANY總是意味着在啟動階段你ifconfig的第一個接口，並且如果他是一個SLIP或者PPP接口的話，他還必須是up的來讓你訪問到你自己的域名服務器。接下來決定了既然推薦的實踐是總是通過127.0.0.1為所有你的網絡接口安裝本地靜態路由，我們可以通過不改動代碼來解決這個問題。
配置文件應該一直被使用，因為他是指定一個默認域名的唯一方法。如果你正在你的本地機器上運行一個服務器，你應該寫nameserver 0.0.0.0或者nameserver 127.0.0.1在配置文件中。
完全正確返回0 錯誤返回-1
/*
* These three fields used to be statically initialized. This made
* it hard to use this code in a shared library. It is necessary,
* now that we're doing dynamic initialization here, that we preserve
* the old semantics: if an application modifies one of these three
* fields of _res before res_init() is called, res_init() will not
* alter them. Of course, if an application is setting them to
* _zero_ before calling res_init(), hoping to override what used
* to be the static default, we can't detect it and unexpected results
* will follow. Zero for any of these fields would make no sense,
* so one can safely assume that the applications were already getting
* unexpected results.
*
* _res.options is tricky since some apps were known to diddle the bits
* before res_init() was first called. We can't replicate that semantic
* with dynamic initialization (they may have turned bits off that are
* set in RES_DEFAULT). Our solution is to declare such applications
* "broken". They could fool us by setting RES_INIT but none do (yet).
*/
這三個域被用來靜態的初始化。這使得它很難在共享庫中使用。這是必要的，既然我們正在做動態初始化那么我們，那我們就要保持舊的語義：如果應用程序在調用res_init（）之前修改了這三個_res域的一個的話，res_init()將不會改變他們。當然，如果一個應用程序在調用 res_init()之前將他們設置成0，來希望覆蓋靜態默認的配置，那么我們將不會檢測到並且意想不到的結果會發生。0對於這些域來說沒有任何意義，所以你可以大膽的假設應用已經得到不可預料的結果。
_res.options有點棘手因為有些apps已知已經在調用res_init之前顛倒了這些bits。我們不能用動態初始化來復制這些語義（他們可能在RES_DEFAULT將這些位關閉了）。我們的解決方案是聲明這些應用程序broken。他們可以愚弄我們通過設置RES_INIT，但是沒有人這樣做。