166 lines
12 KiB
HTML
166 lines
12 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>getenv, getenv_s</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_program_getenv skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading">getenv, getenv_s</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-program.html">program</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><stdlib.h></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl">
|
|
<td>
|
|
<div><span class="mw-geshi c source-c"><span class="kw4">char</span> <span class="sy2">*</span>getenv<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>name <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(1)</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-c11">
|
|
<td>
|
|
<div><span class="mw-geshi c source-c">errno_t getenv_s<span class="br0">(</span> <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> <span class="sy2">*</span><span class="kw4">restrict</span> len, <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> value,<br />
|
|
rsize_t valuesz, <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> name <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(2)</td>
|
|
<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="t-li1"><span class="t-li">1)</span> 在宿主指定的环境列表中，查找名为 <code>name</code> 的环境变量，并返回关联到匹配环境变量的字符串。环境变量的集合及修改它的方法是实现定义的。</div>
|
|
<div class="t-li1"><span class="t-li"></span> 此函数不保证是线程安全的。另一个对 <code>getenv</code> 的调用，和对 POSIX 函数 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/setenv.html">setenv()</a> 、 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/unsetenv.html">unsetenv()</a> 及<a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/putenv.html">putenv()</a> 的调用，可能会使先前的调用返回的指针非法，或者修改先前调用所得的字符串。</div>
|
|
<div class="t-li1"><span class="t-li"></span> 修改 <code>getenv</code> 返回的字符串会引起未定义行为。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了将环境变量的值写入用户提供的缓冲区 <code>value</code> （除非它为 <code>NULL</code> ），而且将写入的字节数存储于用户提供的位置 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span>len</span></span> （除非它为 <code>NULL</code> ）。若环境变量未设置于环境中，则 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span>len</span></span> 会被写入零（除非是 <code>NULL</code> ），且 <span class="t-c"><span class="mw-geshi c source-c"><span class="st0">'<span class="es5">\0</span>'</span></span></span> 会被写入 <span class="t-c"><span class="mw-geshi c source-c">value<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span></span></span> （除非是 <code>NULL</code> ）。另外，在运行时检测下列错误并调用当前安装的<a href="c-error-set_constraint_handler_s.html">制约处理</a>函数
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li><code>name</code> 是空指针</li>
|
|
<li><code>valuesz</code> 大于 <span class="t-lc">RSIZE_MAX</span></li>
|
|
<li><code>value</code> 是空指针且 <code>valuesz</code> 非零</li>
|
|
</ul>
|
|
</dd>
|
|
<dd>同所有边界检查函数， <code>getenv_s</code> 仅若实现定义 <span class="t-c"><span class="mw-geshi c source-c">__STDC_LIB_EXT1__</span></span> 且用户在包含 <code>stdlib.h</code> 前定义 <span class="t-c"><span class="mw-geshi c source-c">__STDC_WANT_LIB_EXT1__</span></span> 为整数常量 1 才保证可用。</dd>
|
|
</dl>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<table class="t-par-begin">
|
|
<tr class="t-par">
|
|
<td>name</td>
|
|
<td>-</td>
|
|
<td>标识要查找的环境变量名称的空终止字符串</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>len</td>
|
|
<td>-</td>
|
|
<td>指向用户提供的位置， <code>getenv_s</code> 将会在其中存储环境变量的长度</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>value</td>
|
|
<td>-</td>
|
|
<td>指向用户提供的字符数组， <code>getenv_s</code> 将会在其中存储环境变量内容</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>valuesz</td>
|
|
<td>-</td>
|
|
<td>允许 <code>getenv_s</code> 对目标写入的最大字节数（缓冲区大小）</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<div class="t-li1"><span class="t-li">1)</span> 标识环境变量的值的字符串，若找不到该环境变量则为空指针。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> 若找到环境变量则为零，若找不到该环境变量或发生运行时强制违规，则为非零。错误的情况下，将 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span>len</span></span> 写入零（除非 <code>len</code> 为空指针）。</div>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>POSIX 系统上，亦可通过全局变量 <code>environ</code> 访问<a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08">环境变量</a>，它于 <code><unistd.h></code> 中声明为 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw2">extern</span> <span class="kw4">char</span> <span class="sy2">**</span>environ<span class="sy4">;</span></span></span> ，并可通过可选的 <a href="c-language-main_function.html"><code>main</code> 函数</a>第三参数 <code>envp</code> 访问。</p>
|
|
<p>以空指针为 <code>value</code> ，以零为 <code>valuesz</code> 调用 <code>getenv_s</code> ，可用于确定保有整个结果所需的缓冲区大小。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="co2">#include <stdio.h></span>
|
|
<span class="co2">#include <stdlib.h></span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>name <span class="sy1">=</span> <span class="st0">"PATH"</span><span class="sy4">;</span>
|
|
<span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>env_p <span class="sy1">=</span> getenv<span class="br0">(</span>name<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">if</span> <span class="br0">(</span>env_p<span class="br0">)</span>
|
|
<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"%s = %s<span class="es1">\n</span>"</span>, name, env_p<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>可能的输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">PATH = /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
|
|
<div class="t-ref-std-11">
|
|
<ul>
|
|
<li>C11 标准（ISO/IEC 9899:2011）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.22.4.6 The getenv function (p: 352-353)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>K.3.6.2.1 The getenv_s function (p: 606-607)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c99">
|
|
<ul>
|
|
<li>C99 标准（ISO/IEC 9899:1999）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.20.4.5 The getenv function (p: 317)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c89">
|
|
<ul>
|
|
<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>4.10.4.4 The getenv function</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin"></table>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|