practical examples use dup or dup2

Question

I know what dup / dup2 does, but I have no idea when it would be used.

Any practical examples?

Thanks.

Answer 1

One example use would be I/O redirection. For this you fork a child process and close the stdin or stdout file descriptors (0 and 1) and then you do a dup() on another filedescriptor of your choice which will now be mapped to the lowest available file descriptor, which is in this case 0 or 1.

Using this you can now exec any child process which is possibly unaware of your application and whenever the child writes on the stdout (or reads from stdin, whatever you configured) the data gets written on the provided filedescriptor instead.

Shells use this to implement commands with pipes, e.g. /bin/ls | more by connecting the stdout of one process to the stdin of the other.

Answer 2

The best scenario to understand dup and dup2 is redirection.
First thing we need to know is that the system has 3 default file ids(or variables indicating output or input sources) that deals with the input and output. They are stdin, stdout, stderr, in integers they are 0,1,2. Most of the functions like fprintf or cout are directly output to stdout.
If we want to redirect the output, one way is give, for example, fprintf function more arguments indicating in and out.
However, there is a more elegant way: we can overwrite the default file ids to make them pointing to the file we want to receive the output. dup and dup2 exactly work in this situation.
Let's start with one simple example now: suppose we want to redirect the output of fprintf to a txt file named "chinaisbetter.txt". First of all we need to open this file

int fw=open("chinaisbetter.txt", O_APPEND|O_WRONLY); 

Then we want stdout to point to "chinaisbetter.txt" by using dup function:

dup2(fw,1); 

Now stdout(1) points to the descriptor of "chinaisbetter.txt" even though it's still 1, but the output is redirected now.
Then you can use printf as normal, but the results will be in the txt file instead of showing directly on the screen:

printf("Are you kidding me? \n"); 

PS:

1. This just gives a intuitive explanation, you may need to check the manpage or detailed information. Actually, we say "copy" here, they are not copying everything.

2. The file id here is referring to the handler of the file. The file descriptor mentioned above is a struct the records file's information.