|DUP(2)||System Calls Manual||DUP(2)|
duplicate an existing file descriptor
duplicates an existing object descriptor and returns its value to the
calling process (newd =
dup(oldd)). The argument
oldd is a small non-negative integer index in the
per-process descriptor table. The value must be less than the size of the
table, which is returned by
new descriptor returned by the call is the lowest numbered descriptor
currently not in use by the process.
The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd are duplicate references to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open(2) call. The close-on-exec flag on the new file descriptor is unset.
value of the new descriptor newd is specified. If this
descriptor is already in use, it is first deallocated as if a
close(2) call had been done
first. When newd equals oldd,
dup2() just returns without affecting the
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.
dup() will fail if:
dup2() will fail if:
are expected to conform to IEEE Std 1003.1-2008
dup() system call first appeared in
Version 3 AT&T UNIX and
dup2() in Version 7 AT&T
|July 17, 2013||OpenBSD-5.5|