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atoi函数的问题及解决方法.html
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<h2 id="问题描述"><a href="#问题描述" class="headerlink" title="问题描述"></a>问题描述</h2><p>在最近的一个项目中需要对传入的字符串参数转换成<code>int</code>类型，并对转换后的值进行判断，看是否在<code>0-7</code>内，最直接的办法就是调用函数：</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><stdlib.h></span></span>
<span class="token keyword">int</span> <span class="token function">atoi</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>nptr<span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<p>但是这个函数有一个明显的问题，对于非法的输入，返回值都为零，而且不进行错误检查。<br>这就造成当函数返回零的时候无法判断输入的是字符<code>0</code>还是非法字符。</p>
<p>如下面的输入和对应的输出：</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token function">atoi</span><span class="token punctuation">(</span><span class="token string">"aaa"</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token number">0</span>
<span class="token function">atoi</span><span class="token punctuation">(</span><span class="token string">"0"</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token number">0</span>
<span class="token function">atoi</span><span class="token punctuation">(</span><span class="token string">"123abc"</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token number">123</span>
<span class="token function">atoi</span><span class="token punctuation">(</span><span class="token string">"abc123"</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token number">0</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p>第一行和第二行输出一样但是输入不一样，造成程序无法区别非法输入和<code>0</code>， 第三行在我们的需求中属于非法输入，但却切断字符返回正常整数。这就造成如果输入的是<code>3a</code>，返回值则为<code>3</code>，和我们期望的返回不一致。</p>
<p>下面给出针对这个项目需求的改进方法。</p>
<hr>
<h2 id="使用atoi转换前进行检查"><a href="#使用atoi转换前进行检查" class="headerlink" title="使用atoi转换前进行检查"></a>使用atoi转换前进行检查</h2><p>既然atoi函数本身的功能不能满足我们的要求，那我们就自己进行检查。</p>
<p>在下面的代码中，我们用返回值来指示传入的字符是否非法，如果非法，返回1，否则返回零。字符串的转换结果使用函数参数返回。</p>
<p>转换函数在第3行检测空指针和空字符；第6行的循环跳过所有前面的空格；第8行检测是否有正负号；第10行的循环遍历剩下的字符，如果有非数字字符，则返回1。否则调用atoi函数进行转换。现在这个函数可以满足我们项目的全部要求。</p>
<p>但是，这个函数的代码比较比较复杂，各种检查不容易理解和维护。进一步的改进请往下看。</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token keyword">int</span> <span class="token function">atoi_c</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span><span class="token operator">*</span> buf<span class="token punctuation">,</span> <span class="token keyword">int</span><span class="token operator">*</span> result<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">int</span> offset <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>buf <span class="token operator">==</span> <span class="token constant">NULL</span> <span class="token operator">||</span> <span class="token operator">*</span>buf <span class="token operator">==</span> <span class="token char">'\0'</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">while</span><span class="token punctuation">(</span>buf<span class="token punctuation">[</span>offset<span class="token punctuation">]</span> <span class="token operator">==</span> <span class="token char">' '</span><span class="token punctuation">)</span>
offset<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>buf<span class="token punctuation">[</span>offset<span class="token punctuation">]</span> <span class="token operator">==</span> <span class="token char">'+'</span> <span class="token operator">||</span> buf<span class="token punctuation">[</span>offset<span class="token punctuation">]</span> <span class="token operator">==</span> <span class="token char">'-'</span><span class="token punctuation">)</span>
offset<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span>buf<span class="token punctuation">[</span>offset<span class="token punctuation">]</span> <span class="token operator">!=</span> <span class="token char">'\0'</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>buf<span class="token punctuation">[</span>offset<span class="token punctuation">]</span> <span class="token operator">></span> <span class="token char">'9'</span> <span class="token operator">||</span> buf<span class="token punctuation">[</span>offset<span class="token punctuation">]</span> <span class="token operator"><</span> <span class="token char">'0'</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
offset<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token operator">*</span>result <span class="token operator">=</span> <span class="token function">atoi</span><span class="token punctuation">(</span>buf<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<hr>
<h2 id="使用strtol检查非法字符"><a href="#使用strtol检查非法字符" class="headerlink" title="使用strtol检查非法字符"></a>使用strtol检查非法字符</h2><p>通过查阅man手册，发现<code>strtol</code>函数可以通过对参数进行检查来判断输入的字符是否是合法的字符。</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><stdlib.h></span></span>
<span class="token keyword">long</span> <span class="token keyword">int</span> <span class="token function">strtol</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>nptr<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span><span class="token operator">*</span>endptr<span class="token punctuation">,</span> <span class="token keyword">int</span> base<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<p>第一个参数就是要转换的字符串；第二个参数非<code>NULL</code>时，储存第一个非法的转换字符；第三个参数是转换的进制。有了第二个参数我们可以通过检查它是不是字符串的最后一个字符(<code>\0</code>)来判断整个字符串是不是合法的。</p>
<p>改进之后的代码如下所示：</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token keyword">int</span> <span class="token function">atoi_c</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span><span class="token operator">*</span> buf<span class="token punctuation">,</span> <span class="token keyword">int</span><span class="token operator">*</span> result<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">char</span><span class="token operator">*</span> invalid<span class="token punctuation">;</span>
<span class="token operator">*</span>result <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token function">strtol</span><span class="token punctuation">(</span>buf<span class="token punctuation">,</span> <span class="token operator">&</span>invalid<span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">*</span>invalid <span class="token operator">!=</span> <span class="token char">'\0'</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>但是这个代码还是有一点问题，当我们将<code>long int</code>类型的值强制转换成<code>int</code>时，就有可能发生溢出，导致最后的转换结果出错。而且我们只需要对<code>0-7</code>的值进行判定，所以使用<code>unsigned int</code>足够了。</p>
<p>我们继续对代码进行改进。</p>
<hr>
<h2 id="使用strtoumax改进代码"><a href="#使用strtoumax改进代码" class="headerlink" title="使用strtoumax改进代码"></a>使用strtoumax改进代码</h2><pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><inttypes.h></span></span>
<span class="token class-name">intmax_t</span> <span class="token function">strtoimax</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>nptr<span class="token punctuation">,</span> <span class="token keyword">char</span> <span class="token operator">*</span><span class="token operator">*</span>endptr<span class="token punctuation">,</span> <span class="token keyword">int</span> base<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<p>在上面的函数原型中，各个参数的作用和<code>strtol</code>的一样但是这个函数有控制溢出的能力，更多的信息可以查看<code>man 3 strtoumax</code> 。</p>
<p>函数的返回值<code>intmax_t</code>在 <code>Ubuntu2004</code> 中的定义是<code>unsigned long int</code>，为了适应返回值，我们可以对返回值先进行比较，如果大于<code>unsigned int</code>的最大值的话，返回错误。最后的代码如下：</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span><span class="token string"><inttypes.h></span></span>
<span class="token keyword">int</span> <span class="token function">atoi_c</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span><span class="token operator">*</span> buf<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span><span class="token operator">*</span> result<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>buf <span class="token operator">==</span> <span class="token constant">NULL</span> <span class="token operator">||</span> <span class="token operator">*</span>buf <span class="token operator">==</span> <span class="token char">'\0'</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">char</span><span class="token operator">*</span> invalid<span class="token punctuation">;</span>
<span class="token class-name">uintmax_t</span> ret<span class="token punctuation">;</span>
ret <span class="token operator">=</span> <span class="token function">strtoumax</span><span class="token punctuation">(</span>buf<span class="token punctuation">,</span> <span class="token operator">&</span>invalid<span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">*</span>invalid <span class="token operator">!=</span> <span class="token char">'\0'</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>ret<span class="token operator">></span> UINT32_MAX<span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token operator">*</span>result <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">unsigned</span> <span class="token keyword">int</span><span class="token punctuation">)</span>ret<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
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<a href="/atom.xml" class="tooltipped" target="_blank" data-tooltip="RSS 订阅" data-position="top" data-delay="50">
<i class="fas fa-rss"></i>
</a>
</div>
</div>
</footer>
<div class="progress-bar"></div>
<!-- 搜索遮罩框 -->
<div id="searchModal" class="modal">
<div class="modal-content">
<div class="search-header">
<span class="title"><i class="fas fa-search"></i> 搜索</span>
<input type="search" id="searchInput" name="s" placeholder="请输入搜索的关键字"
class="search-input">
</div>
<div id="searchResult"></div>
</div>
</div>
<script type="text/javascript">
$(function () {
var searchFunc = function (path, search_id, content_id) {
'use strict';
$.ajax({
url: path,
dataType: "xml",
success: function (xmlResponse) {
// get the contents from search data
var datas = $("entry", xmlResponse).map(function () {
return {
title: $("title", this).text(),
content: $("content", this).text(),
url: $("url", this).text()
};
}).get();
var $input = document.getElementById(search_id);
var $resultContent = document.getElementById(content_id);
$input.addEventListener('input', function () {
var str = '<ul class=\"search-result-list\">';
var keywords = this.value.trim().toLowerCase().split(/[\s\-]+/);
$resultContent.innerHTML = "";
if (this.value.trim().length <= 0) {
return;
}
// perform local searching
datas.forEach(function (data) {
var isMatch = true;
var data_title = data.title.trim().toLowerCase();
var data_content = data.content.trim().replace(/<[^>]+>/g, "").toLowerCase();
var data_url = data.url;
data_url = data_url.indexOf('/') === 0 ? data.url : '/' + data_url;
var index_title = -1;
var index_content = -1;
var first_occur = -1;
// only match artiles with not empty titles and contents
if (data_title !== '' && data_content !== '') {
keywords.forEach(function (keyword, i) {
index_title = data_title.indexOf(keyword);