Linux中文件描述符fd与文件指针FILE*互相转换实例解析
<p>本文研究的主要是linux中文件描述符fd与文件指针file*互相转换的相关内容，具体介绍如下。</p>
<p>
<strong>1.文件描述符fd的定义:</strong>文件描述符在形式上是一个非负整数。实际上，它是一个索引值，指向内核为每一个进程所维护的该进程打开文件的记录表。当程序打开一个现有文件或者创建一个新文件时，内核向进程返回一个文件描述符。在程序设计中，一些涉及底层的程序编写往往会围绕着文件描述符展开。但是文件描述符这一概念往往只适用于unix、linux这样的操作系统。</p>
<p>
<strong>2.文件指针file定义说明文件指针的一般形式为：</strong></p>
<p>
file *指针变量标识符；</p>
<p>
其中file应为大写，它实际上是由系统定义的一个结构，该结构中含有文件名、文件状态和文件当前位置等信息。在编写源程序时不必关心file结构的细节。</p>
<p>
使用系统调用的时候用文件描述符的时候比较多，但是操作比较原始。c库函数在i/o上提供了一些方便的包装（比如格式化i/o、重定向），但是对细节的控制不够。</p>
<p>
如果过度依赖其中的一种只会徒增麻烦，所以知道两者的转换是很有必要的。file*是对fd的封装</p>
<p>
当然，有人会说知道文件路径的话重新打开就是了，但是这会产生竞争条件（race conditions），首先重新打开文件，相当于是2个fd指向同一文件，然后如果在打开的期间文件被删除了又被新建了一个同名文件，2个fd指向的便是不同的文件。</p>
<p>
glibc库提供了两个转换函数fdopen(3)和fileno(3)，都是<stdio.h>中的</p>
<p>
<code>file *fdopen(int fd, const char *mode);<br>
int fileno(file *stream);</code></p>
<p>
ps：为了节省篇幅，还是继续忽略返回值的检查。</p>
<p>
来看看测试吧，是不是我们想的那样。</p>
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<code class="cpp preprocessor">#include <stdio.h></code>
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<code class="cpp preprocessor">#include <unistd.h></code>
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<code class="cpp preprocessor">#include <fcntl.h></code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">main()</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">const</code> <code class="cpp color1 bold">char</code><code class="cpp plain">* filename = </code><code class="cpp string">"new.txt"</code><code class="cpp plain">;</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">fd = open(filename, o_rdwr | o_creat, s_irusr | s_iwusr);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">file* fp = fdopen(fd, </code><code class="cpp string">"w+"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">fd2 = fileno(fp);</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"fd=%d | fd2=%d\n"</code><code class="cpp plain">, fd, fd2);</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">fclose</code><code class="cpp plain">(fp);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">close(fd);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
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<code class="cpp plain">}</code>
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<code class="cpp plain">$ gcc test.c </code>
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<code class="cpp plain">$ ./a.out </code>
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<code class="cpp plain">fd=3 | fd2=3</code>
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<p>
参考fileno手册：</p>
<blockquote>
<p>
the function fileno() examines the argument stream and returns its integer descriptor.</p>
</blockquote>
<p>
file是对fd的封装，fileno()是直接取得被封装的fd，因此并未创建新的fd指向该文件。</p>
<p>
<strong>参考fdopen手册：</strong></p>
<blockquote>
<p>
the fdopen() function associates a stream with the existing file descriptor, fd. the mode of<br>
the stream (one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the<br>
mode of the file descriptor.</p>
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<p>
fdopen()是讲流（file对象）与已存在的文件描述符fd进行关联，因此也是未创建新的fd。值得注意的是，file指针的模式（mode）必须与文件描述符的模式兼容。</p>
<p>
关于mode参数先搁置会儿，目前我们知道的是，使用fileno和fdopen进行转换，都是在原有的fd上进行操作，并未产生新的fd。那么，再次审视刚才的代码，是否发现了问题？</p>
<p>
我们来检查下close(fd)的返回值，把close(fd)改成下列代码</p>
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<code class="cpp keyword bold">if</code> <code class="cpp plain">(-1 == close(fd)) {</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">perror</code><code class="cpp plain">(</code><code class="cpp string">"close"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(1);</code>
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<code class="cpp plain">}</code>
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<code class="cpp plain">$ gcc test.c </code>
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<code class="cpp plain">$ ./a.out </code>
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<code class="cpp plain">close: bad file descriptor</code>
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<p>
没错，fclose在关闭文件指针的时候，内部其实也关闭了文件描述符（否则资源就泄露了），既然这里fp内部的文件描述符和fd是同一个，当fp被关闭时，fd也被关闭了，再次关闭fd就会出现“损坏的文件描述符”错误。</p>
<p>
ok，现在回顾下fopen的第2个参数，又r/r+/w/w+/a/a+一共6种设置（windows平台的rb/rb+/wb/wb+暂且不谈），对比linux手册我将对应的open设置列出来</p>
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<img title="Linux中文件描述符fd与文件指针FILE*互相转换实例解析" alt="Linux中文件描述符fd与文件指针FILE*互相转换实例解析" id="theimg" src="https://zhuji.jb51.net/uploads/img/202305/0bd9c7c431f1730307429fd363322f65.jpg"></p>
<p>
<strong>依然是进行测试，修改fd_mode和fp_mode，看看实验结果</strong></p>
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<code class="cpp preprocessor">#include <stdio.h></code>
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<code class="cpp preprocessor">#include <stdlib.h></code>
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<code class="cpp preprocessor">#include <unistd.h></code>
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<code class="cpp keyword bold">const</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">security = s_irusr | s_iwusr;</code>
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<code class="cpp keyword bold">const</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">fd_mode = o_rdwr | o_creat | o_trunc;</code>
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<code class="cpp keyword bold">const</code> <code class="cpp color1 bold">char</code><code class="cpp plain">* fp_mode = </code><code class="cpp string">"r"</code><code class="cpp plain">;</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">fd = open(</code><code class="cpp string">"new.txt"</code><code class="cpp plain">, fd_mode, security);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">file* fp = fdopen(fd, fp_mode);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(fp == null) {</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">perror</code><code class="cpp plain">(</code><code class="cpp string">"fdopen"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(1);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code>
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<code class="cpp spaces"> </code><code class="cpp plain">close(fd);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
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<code class="cpp plain">}</code>
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<p>
在fd_mode等价于"w+"时，fp_mode的6种设置（r/r+/w/w+/a/a+）均返回非空指针。</p>
<p>
在fd_mode等价于"w"时，fp_mode6种设置只有"a"和"w"返回非空指针。</p>
<p>
继续尝试"r"/"r+"/"a"/"a+"的设置，可以发现所谓“兼容”只与读写权限有关，o_rdwr兼容o_rdonly和o_wronly，而后两者则只与自身兼容。</p>
<p>
有意思的是o_append（在末尾添加）和o_trunc（截断文件从头添加）也兼容。</p>
<blockquote>
<p>
the file position indicator of the new stream is set to that<br>
belonging to fd, and the error and end-of-file indicators are cleared. modes "w" or "w+" do<br>
not cause truncation of the file. the file descriptor is not dup'ed, and will be closed when<br>
the stream created by fdopen() is closed.</p>
</blockquote>
<p>
继续查看fdopen的手册内容，可以看到"w"和"w+"在这里不会导致文件截断。</p>
<p>
后一句也印证了我们前面的实验结果：文件描述符不会被复制，文件指针被关闭时文件描述符也会被关闭。</p>
<p>
ps：其实fdopen的手册上还有最后一句：the result of applying fdopen() to a shared memory object is undefined.</p>
<p>
将fdopen用于共享内存对象的结果是未定义的。</p>
<p>
<span><strong>总结</strong></span></p>
<p>
以上就是本文关于linux中文件描述符fd与文件指针file*互相转换实例解析的全部内容，希望对大家有所帮助。感兴趣的朋友可以继续参阅本站其他相关专题，如有不足之处，欢迎留言指出。感谢朋友们对本站的支持！</p>
<p>
原文链接：http://www.cnblogs.com/Harley-Quinn/p/7481593.html</p>
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