What do the dup() and dup2() systems really do?

Question

What do the dup() and dup2() system calls really do? How would I use them in practice?

What part don't you understand after reading the man pages and searching? — Matthew Flaschen, Nov 13 '10 at 05:34
i was unable to understand how to use in programming and where to use — user484457, Nov 13 '10 at 05:35
Have you looked at http://stackoverflow.com/questions/1720535/practical-examples-use-dup-or-dup2 or http://stackoverflow.com/questions/3918962/unix-c-dup2-question ? — Zeke, Nov 13 '10 at 05:39

score 5 · Accepted Answer · answered Nov 13 '10 at 05:43

5

Both make a new file descriptor corresponding to an existing open file description. Most properties between the old and new fd (like position) are shared; the only property I can think of that's not shared is the close-on-exec flag. The difference between dup and dup2 is that dup assigns the lowest available file descriptor number, while dup2 lets you choose the file descriptor number that will be assigned and atomically closes and replaces it if it's already taken.

answered Nov 13 '10 at 05:43

R.. GitHub STOP HELPING ICE

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Also note that `fcntl` with the `F_DUPFD` command behaves similarly to `dup2`, but does not clobber an existing fd, so it can safely be used in threaded programs when trying to create a particular fd number that's not already taken. – R.. GitHub STOP HELPING ICE Jan 07 '12 at 04:23
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Besides, if the file referenced by `newfd` is already open, it will be closed before the duplication is performed. – Soner from The Ottoman Empire Mar 30 '19 at 12:40
@snr: Not "before". Rather *atomically with respect to*. – R.. GitHub STOP HELPING ICE Mar 30 '19 at 19:11

What do the dup() and dup2() systems really do?

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