blocks|key|3144244|text|看到这个page，stdout可以别名为dup(1)...|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3144245|entityMap|0|LINK|mutability|MUTABLE|url|http://codewiki.wikidot.com/c%253asystem-calls%253adup^0|K|6|4|4|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@$9|Q|A|R|1|S]]|E|$]]|$1|F|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

see this <a href="http://codewiki.wikidot.com/c%3asystem-calls%3adup" rel="nofollow">page</a>, stdout can be aliased as <code>dup(1)</code>...

blocks|key|251837|text|假设您正在编写一个shell程序，并且希望在要运行的程序中重定向stdin和stdout。它可能看起来像这样：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|251838|fdin+=+open(infile,+O_RDONLY);
fdout+=+open(outfile,+O_WRONLY);
//+Check+for+errors,+send+messages+to+stdout.
...
int+pid+=+fork(0);
if(pid+==+0)+{
++++close(0);
++++dup(fdin);
++++close(fdin);
++++close(1);
++++dup(fdout);
++++close(fdout);
++++execvp(program,+argv);
}
//+Parent+process+cleans+up,+maybe+waits+for+child.
...|code-block|syntax|javascript|251839|dup2()是一种更方便的方法，close()+dup()可以替换为：|251840|dup2(fdin,+0);
dup2(fdout,+1);|251841|这样做的原因是您希望将错误报告给stdout+(或stderr)，这样就不能在子进程中关闭它们并打开一个新文件。其次，如果open()调用返回错误，那么执行fork将是一种浪费。|251842|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Let's say you're writing a shell program and you want to redirect stdin and stdout in a program you want to run. It could look something like this:

<pre><code>fdin = open(infile, O_RDONLY);
fdout = open(outfile, O_WRONLY);
// Check for errors, send messages to stdout.
...
int pid = fork(0);
if(pid == 0) {
 close(0);
 dup(fdin);
 close(fdin);
 close(1);
 dup(fdout);
 close(fdout);
 execvp(program, argv);
}
// Parent process cleans up, maybe waits for child.
...
</code></pre>

dup2() is a little more convenient way to do it the close() dup() can be replaced by:

<pre><code>dup2(fdin, 0);
dup2(fdout, 1);
</code></pre>

The reason why you want to do this is that you want to report errors to stdout (or stderr) so you can't just close them and open a new file in the child process. Secondly, it would be a waste to do the fork if either open() call returned an error.

blocks|key|248270|text|文件描述符不仅仅是一个数字。它还带有各种半隐藏状态(无论它是否打开，它引用的是哪个文件描述，以及一些标志)。dup会复制此信息，因此您可以分别关闭这两个描述符。fd=fd2不需要。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|248271|entityMap^0|1I|3|28|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

A file descriptor is a bit more than a number. It also carries various semi-hidden state with it (whether it's open or not, to which file description it refers, and also some flags). <code>dup</code> duplicates this information, so you can e.g. close the two descriptors independently. <code>fd=fd2</code> does not.

blocks|key|2929706|text|我只是想在试验了一下之后对第二个问题做出回应。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2929707|答案是，是的，。如果stdin、stdout或stderr被关闭，则您创建的文件描述符可以采用值0、1、2。|offset|length|style|BOLD|2929708|示例：|2929709|close(1);+++++//closing+stdout
newfd=dup(1);+//newfd+takes+value+of+least+available+fd+number|code-block|syntax|javascript|2929710|文件描述符发生这种情况的地方：|2929711|0+stdin+++++.--------------.+++++0+stdin+++++.--------------.+++++0+stdin
1+stdout+++=%7C+++close(1)+++:=>+++2+stderr+++=%7C+newfd=dup(1)+:=>+++1+newfd
2+stderr++++'--------------'+++++++++++++++++'--------------'+++++2+stderr|2929712|entityMap^0|0|3|4|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|Z|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|L|7|11|8|@]|9|@]|A|$M|N]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

Just wanted to respond to myself on the second question after experimenting a bit.

The answer is YES. A file descriptor that you make can take a value 0, 1, 2 if stdin, stdout or stderr are closed.

Example:

<pre><code>close(1); //closing stdout
newfd=dup(1); //newfd takes value of least available fd number
</code></pre>

Where this happens to file descriptors:

<pre><code>0 stdin .--------------. 0 stdin .--------------. 0 stdin
1 stdout =| close(1) :=&gt; 2 stderr =| newfd=dup(1) :=&gt; 1 newfd
2 stderr '--------------' '--------------' 2 stderr
</code></pre>

blocks|key|3144392|text|dup()最重要的一点是，它返回新文件描述符可用的最小整数。这是重定向的基础：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3144393|int+fd_redirect_to+=+open("file",+O_CREAT);
close(1);+/*+stdout+*/
int+fd_to_redirect+=+dup(fd_redirect_to);+/*+magically+returns+1:+stdout+*/
close(fd_redirect_to);+/*+we+don't+need+this+*/|code-block|syntax|javascript|3144394|在此之后，任何写入文件描述符1+(stdout)的内容都会神奇地进入"+file“。|3144395|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The single most important thing about dup() is it returns the smallest integer available for a new file descriptor. That's the basis of redirection:

<pre><code>int fd_redirect_to = open("file", O_CREAT);
close(1); /* stdout */
int fd_to_redirect = dup(fd_redirect_to); /* magically returns 1: stdout */
close(fd_redirect_to); /* we don't need this */
</code></pre>

After this anything written to file descriptor 1 (stdout), magically goes into "file".

blocks|key|3144416|text|这只是一个关于“复制标准输出”的提示。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3144417|在某些Unix系统上(但不是GNU/Linux)|3144418|fd+=+open("/dev/fd/1",+O_WRONLY);|code-block|syntax|javascript|3144419|相当于：|3144420|fd+=+dup(1);|3144421|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Just a tip about "duplicating standard output".

On some Unix Systems (but not GNU/Linux)

<pre><code>fd = open("/dev/fd/1", O_WRONLY);
</code></pre>

it is equivalent to:

<pre><code>fd = dup(1);
</code></pre>

blocks|key|253413|text|dup()和dup2()系统调用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|253414|·dup()系统调用复制打开的文件描述符，并返回新的文件描述符。|253415|·新文件描述符具有与原始文件描述符相同的以下属性:+1.引用相同的打开文件或管道。2.具有相同的文件指针--即，两个文件描述符共享一个文件指针。3.具有相同的访问模式，无论是读取、写入还是读取和写入。|253416|·dup()保证返回具有最小整数值的文件描述符available.It是因为该功能返回进程完成I/O重定向的可用的最低未使用文件描述符。|253417|集成重复数据(File_descriptor)|253418|int+dup2(file_descriptor1，file_descriptor2)|253419|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|P|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Q|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|R|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|S|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|M|$]]

dup() and dup2() system call

•The dup() system call duplicates an open file descriptor and returns the new file 
descriptor.

•The new file descriptor has the following properties in common with
the original 
file descriptor:
1. refers to the same open file or pipe.
2. has the same file pointer -- that is, both file descriptors share one file pointer.
3. has the same access mode, whether read, write, or read and write.

• dup() is guaranteed to return a file descriptor with the lowest integer value available.It is because of this feature of returning the lowest unused file descriptor available that processes accomplish I/O redirection.

int dup(file_descriptor)

int dup2(file_descriptor1, file_descriptor2)

I know that dup, dup2, dup3 "create a copy of the file descriptor oldfd"(from man pages). However I can't digest it.

As I know file descriptors are just numbers to keep track of file locations and their direction(input/output). Wouldn't it be easier to just

<pre><code>fd=fd2;
</code></pre>

Whenever we want to duplicate a file descriptor?

And something else..

<blockquote>
 dup() uses the lowest-numbered unused descriptor for the new descriptor.
</blockquote>

Does that mean that it can also take as value stdin, stdout or stderr if we assume that we have close()-ed one of those?

Can someone explain what dup() in C does?

Linux

Unix

我知道dup、dup2、dup3“创建文件描述符oldfd的副本”(来自手册页)。但是我不能消化它。据我所知，文件描述符只是号，用于跟踪文件位置及其方向(输入/输出)的。不是更容易些吗fd=fd2;什么时候我们想复制一个文件描述符？还有其他的..。 dup()对新的描述符使用编号最小的未使用描述符。这是否意味着如果我们...

问有人能解释一下C中的dup()是做什么的吗？
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