DUP(2) UNIX Programmer's Manual DUP(2)
dup, dup2 - duplicate a descriptor
S
\bSY
\bYN
\bNO
\bOP
\bPS
\bSI
\bIS
\bS
n
\bne
\bew
\bwd
\bd =
\b= d
\bdu
\bup
\bp(
\b(o
\bol
\bld
\bdd
\bd)
\b)
i
\bin
\bnt
\bt n
\bne
\bew
\bwd
\bd,
\b, o
\bol
\bld
\bdd
\bd;
\b;
d
\bdu
\bup
\bp2
\b2(
\b(o
\bol
\bld
\bdd
\bd,
\b, n
\bne
\bew
\bwd
\bd)
\b)
i
\bin
\bnt
\bt o
\bol
\bld
\bdd
\bd,
\b, n
\bne
\bew
\bwd
\bd;
\b;
D
\bDE
\bES
\bSC
\bCR
\bRI
\bIP
\bPT
\bTI
\bIO
\bON
\bN
_
\bD_
\bu_
\bp duplicates an existing object descriptor. The argument
_
\bo_
\bl_
\bd_
\bd 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 _
\bg_
\be_
\bt_
\bd_
\bt_
\ba_
\bb_
\bl_
\be_
\bs_
\bi_
\bz_
\be(2).
The new descriptor returned by the call, _
\bn_
\be_
\bw_
\bd, is the lowest
numbered descriptor that is not currently in use by the pro-
The object referenced by the descriptor does not distinguish
between references using _
\bo_
\bl_
\bd_
\bd and _
\bn_
\be_
\bw_
\bd in any way. Thus if
_
\bn_
\be_
\bw_
\bd and _
\bo_
\bl_
\bd_
\bd are duplicate references to an open file,
_
\br_
\be_
\ba_
\bd(2), _
\bw_
\br_
\bi_
\bt_
\be(2) and _
\bl_
\bs_
\be_
\be_
\bk(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 _
\bo_
\bp_
\be_
\bn(2) call. The close-on-exec flag on the new
file descriptor is unset.
In the second form of the call, the value of _
\bn_
\be_
\bw_
\bd desired is
specified. If this descriptor is already in use, the
descriptor is first deallocated as if a _
\bc_
\bl_
\bo_
\bs_
\be(2) call had
R
\bRE
\bET
\bTU
\bUR
\bRN
\bN V
\bVA
\bAL
\bLU
\bUE
\bE
The value -1 is returned if an error occurs in either call.
The external variable _
\be_
\br_
\br_
\bn_
\bo indicates the cause of the
_
\bD_
\bu_
\bp and _
\bd_
\bu_
\bp_
\b2 fail if:
[EBADF] _
\bO_
\bl_
\bd_
\bd or _
\bn_
\be_
\bw_
\bd is not a valid active descriptor
[EMFILE] Too many descriptors are active.
S
\bSE
\bEE
\bE A
\bAL
\bLS
\bSO
\bO
accept(2), open(2), close(2), fcntl(2), pipe(2), socket(2),
socketpair(2), getdtablesize(2)
Printed 7/9/88 May 13, 1986 1
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