在类Unix操作系统里面，。dup2和dup都通过系统调用来产生一份file descriptor
的拷贝。
dup对我来说还很简单
int dup(int
filedes);
dup2就有点犯迷糊了
int dup2(int
filedes1,int
filedes2);
其实这样declaration更好
int
dup2(int oldfd,int
newfd)
下面是apue给出的解释
With dup2, we
specify the value of the new descriptor with the fd2
argument.
If fd2 is already open, it is first closed. If fd
equals fd2,t hen dup2 re turns fd2 without closing
it.
Otherwise, theFD_CLOEXECfile descriptor flag is cleared
forfd2, so that fd2 is left open if the process calls
exec
我就比较疑惑，如果newfd是STDIN_FILENO,STDOUT_FILENO,STDERR_FILENO，(0，1，2)三个中的一个呢?
dup2是返回错误还是
“fd2 is already open, it is first closed”呢?
Just test
it.
#include
#include"fcntl.h"
#include
int
main()
{
int
file_descriptor;
if((file_descriptor =
open("./text.t",O_RDWR)) <
0)
{
printf("error\n");
}
else
{
printf("file
descriptor is
%d\n",file_descriptor);
}
printf("dup2
return value:%d\n dup return
value\
%d\n",dup2(file_descriptor,0),dup(file_descriptor));
printf("dup2
return value:%d\ndup return
value\
%d\n",dup2(file_descriptor,1),dup(file_descriptor));
printf("dup2
return value:%d\ndup return
value\
%d\n",dup2(file_descriptor,2),dup(file_descriptor));
close(file_descriptor);
return
0;
}
Aha! Something interesting
happened.
运行结果：
file descriptor is
3
dup2 return value:0
dup return value
4
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。。。
问题都在第二个printf语句中调用的dup2
www.jx-jf.com
dup2的第二个参数是1，1是什么?STDIN_FILENO宏的本质就是1.这里还是那句话“fd2 is
already open, it is first
closed”
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
www.yzyedu.com
So。。。dup2非常“老实本分"的把STDOUT_FILENO在第二printf的时候关闭了
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!!
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。
code：
#include
#include"fcntl.h"
#include
int
main()
{
int
file_descriptor;
if((file_descriptor =
open("./text.t",O_RDWR)) <
0)
{
printf("error\n");
}
else
{
printf("file
descriptor is
%d\n",file_descriptor);
}
printf("dup2
return value:%d\n dup return
value\
%d\n",dup2(file_descriptor,0),dup(file_descriptor));
printf("dup2
return value:%d\ndup return
value\
%d\n",dup2(file_descriptor,10),dup(file_descriptor));
printf("dup2
return value:%d\ndup return
value\
%d\n",dup2(file_descriptor,11),dup(file_descriptor));
close(file_descriptor);
return
0;
}
运行结果：
file
descriptor is 3
dup2 return
value:0
dup return value 4
dup2 return
value:10
dup return value 5
dup2
return value:11
dup return value
6
这个时候就很清楚了。
Similarly,the
call
dup2(fd, fd2);
is equivalent
to
close(fd2);
fcntl(fd, F_DUPFD,
fd2);
当然也不完全相同，不同点如下：
1. dup2 is an
atomic operation, whereas the alternate form involves two
function
calls. It is possible in the latter case to have a
signal catcher called between the
closeand the fcntlthat
could modify the file descriptors. (W edescribe
signals in
Chapter 10.) The same problem could occur if a different
thread
changes the file descriptors. (Wedescribe threads in
Chapter 11.)
2. Ther eare s ome errnodifferences between
dup2 and fcntl.
用dup2和dup产生一份file descriptor 的拷贝,布布扣,bubuko.com
用dup2和dup产生一份file descriptor 的拷贝
原文：http://www.cnblogs.com/tianchaoy/p/3622414.html