dup2这家伙

int dup(int filedes);

int dup2(int filedes1,int filedes2);

int dup2(int oldfd,int newfd)

#include<apue.h>
#include"fcntl.h"
#include<stdio.h>
int main()
{
        int file_descriptor;
        if((file_descriptor = open("./text.t",O_RDWR)) <br>
Aha! Something interesting happened.

<p>
运行结果：</p>
<p>
file descriptor is 3<br>
dup2 return value:0<br>
 dup return value 4<br>
</p>
<p>
<span>以上就是这段code的运行结果。嗯。。。我有三个printf语句，为啥米只答应了第一个，WTF。。。</span></p>
<p>
<span>问题都在第二个printf语句中调用的dup2</span></p>
<p>
<span> dup2的第二个参数是1，1是什么？STDOUT_FILENO宏的本质就是1.这里还是那句话<span>“fd2 is already open, it is first closed”</span><br>
</span></p>
<p>
<span>三个标准流在文件进程开始的时候肯定是都被打开的，already open是一定的事情。</span></p>
<p>
<span>So。。。dup2非常“老实本分"的把STDOUT_FILENO在第二printf的时候关闭了</span></p>
<p>
<span>这个时候printf的输出就不会被打印在标准输出设备上面了，因为STDOUT_FILENO已经被关闭了！！</span></p>
<p>
<span>如果这个时候只要把第二个printf里面dup2的第二个参数改成大于2的数字就没事了。</span></p>
<p>
code：</p>
<p>
</p>
<pre class="brush:php;toolbar:false">#include<apue.h>
#include"fcntl.h"
#include<stdio.h>
int main()
{
        int file_descriptor;
        if((file_descriptor = open("./text.t",O_RDWR)) 

<p>
<span>运行结果：</span></p>
<p>
file descriptor is 3<br>
dup2 return value:0<br>
dup return value 4<br>
dup2 return value:10<br>
dup return value 5<br>
dup2 return value:11<br>
dup return value 6<br>
</p>
<p>
<span>这个时候就很清楚了。</span></p>
<p>
<span>Similarly,the call<br>
dup2(fd, fd2);<br>
is equivalent to<br>
close(fd2);<br>
fcntl(fd, F_DUPFD, fd2);<br>
</span></p>
<p>
<span>当然也不完全相同，不同点如下：</span></p>
<p>
<span><br>
</span></p>
<blockquote>
<p>
<span><strong>1. dup2 is an atomic operation, whereas the alternate form involves two function</strong></span></p>
<p>
<span><strong>calls.  It is possible in the latter case to have a signal catcher called between the</strong></span></p>
<p>
<span><strong>close and  the fcntl that  could  modify  the  file  descriptors. (We describe</strong></span></p>
<p>
<span><strong>signals  in  Chapter  10.) The  same  problem  could  occur  if  a  different  thread</strong></span></p>
<p>
<span><strong>changes the file descriptors. (We describe threads in Chapter 11.)</strong></span></p>
<p>
<span><strong><br>
</strong></span></p>
<p>
<span><strong>2.  There are some errno differences between dup2 and fcntl.</strong></span></p>
</blockquote>
<p>
<span></span></p>
<p>
<span></span></p>
<p>
<br>
</p>
<p>
update 2014.04.10</p>
<p>
<span>今天有了“更深”的认识吧</span></p>
<pre class="brush:php;toolbar:false"><span>int dup2(int filedes1,int filedes2);</span>