Visit Our UNIX and Linux User Community


dup()


 
Thread Tools Search this Thread
Top Forums Programming dup()
# 1  
Old 10-30-2001
Data dup()

Would anyone be so kind to explain to me the function of dup() in UNIX? As far as I am concerned, it duplicates a file descriptor. Under what circumstances would we need to duplicate a file descriptor in a UNIX environment?

Thank you.

vinchen
# 2  
Old 10-30-2001
For example, think about inetd. It has dozens of fd's open. It gets a connection for telnet on one them. So it forks a child. The child must exec telnetd, but first that socket must be duped onto fd's 0 1 and 2.

And lots of times you want to open a file on fd 1 and dup it to fd 2. Opening the file twice would result in two file table entries and then fd 1 and fd 2 would be stepping on each other. Anytime you see "2>&1" in a shell script you're asking the shell to dup an fd.

Anyway, few people actually use dup() anymore. dup2() or fnctl() can dup an fd with more control over the target fd.
# 3  
Old 10-30-2001
dup in unix

hi ..,

You are right, its used to duplicate the file discriptor.

the application demands the situation for example,

you have a client to pull data from a server on a different m/c and
you want your clients to run as daemon processes,
you can not associate any terminal device associated for daemon processes.
In this case, you open a null file discriptor with
open("/dev/null",O_RDONLY)
and dup this discriptor for stdin , stdout and stderr.
this prevents your processes from directly writing or reading from stdio.

I hope this might help you a bit in understanding.

thanks
reddyb
# 4  
Old 10-30-2001
Power

Thank you very much people.

Smilie

Previous Thread | Next Thread
Test Your Knowledge in Computers #570
Difficulty: Medium
The C programming language allows you to pass a parameter to a function by providing its memory address instead of the value stored in it.
True or False?

7 More Discussions You Might Find Interesting

1. UNIX and Linux Applications

Deja-dup make my / full. So I cannot restore my back up

The problematic directory is the following: /root/.cache/deja-dup This directory grows until my "/" is full and then the restoring activity fails. I already tried to create a symbolic link with origin another partition where I have more space. However during the restoring activity ... (4 Replies)
Discussion started by: puertas12
4 Replies

2. Shell Programming and Scripting

How to count dup records in file?

Hi Gurus, I need to count the duplicate records in file file abc abc def ghi ghi jkl I want to get below result: abc ,2 abc, 2 def ,1 ghi ,2 ghi, 2 jkl ,1 or abc ,2 def ,1 (3 Replies)
Discussion started by: ken6503
3 Replies

3. Shell Programming and Scripting

Identify duplicates and update the last 2 digits to 0 for both the Orig and Dup

Hi, I have a requirement where I have to identify duplicates from a file based on the first 6 chars (It is fixed width file of 12 chars length) and whenever a duplicate row is found, its original and duplicate row's last 2 chars should be updated to all 0's if they are not same. (I mean last 2... (3 Replies)
Discussion started by: farawaydsky
3 Replies

4. Red Hat

ping error (DUP!)

Ntop is running on redhat. But It gives DUP! error while pinging to any places I dont know why DUP! error is occured. # ping google.com PING google.com (74.125.39.147) 56(84) bytes of data. 64 bytes from fx-in-f147.1e100.net (74.125.39.147): icmp_seq=1 ttl=44 time=54.1 ms 64 bytes from... (6 Replies)
Discussion started by: getrue
6 Replies

5. Programming

dup()

when i want to replace standard output with output file int out; out = open("out", O_WRONLY)p; dup2(out,1); What Shall I do in case of appending??? I am using here O_WRONLY TO WRITE.BUT IF i wanna append, whats the word? (5 Replies)
Discussion started by: joey
5 Replies

6. Programming

Understanding the purpose of dup/dup2

I'm having difficulty understanding the purposes of using dup/dup2 when involving forks. for example, if we call fork() once, that is, we are creating a child process. In what cases would we need to use dup or dup2 to duplicate the file descriptors for standard output and standard error? What... (1 Reply)
Discussion started by: Yifan_Guo
1 Replies

7. Programming

fork() and dup()

I have met this code: switch(fork()) { case 0: close(1); dup(p); close(p); close(p); execvp(<whatever>); perror("Exec failed"); } Can anyone tell me what this piece of code does? Thx alot.. (1 Reply)
Discussion started by: AkumaTay
1 Replies

Featured Tech Videos