dup
NAME
dup, dup2 - duplicate a file descriptor
SYNOPSIS
#include <unistd.h> I int dup(int oldfd ); I int dup2(int oldfd , int newfd );
DESCRIPTION
R dup ()
and
R dup2 ()
create a copy of the file descriptor
R oldfd .
After a successful return from
R dup ()
or
R dup2 (),
the old and new file descriptors may be used interchangeably.
They refer to the same open file description (see
open(2))
and thus share file offset and file status flags;
for example, if the file offset is modified by using
lseek(2)
on one of the descriptors, the offset is also changed for the other.
The two descriptors do not share file descriptor flags
(the close-on-exec flag).
The close-on-exec flag
(FD_CLOEXEC;
see
fcntl(2))
for the duplicate descriptor is off.
R dup ()
uses the lowest-numbered unused descriptor for the new descriptor.
R dup2 ()
makes newfd be the copy of oldfd, closing newfd
first if necessary.
RETURN VALUE
R dup ()
and
R dup2 ()
return the new descriptor, or -1 if an error occurred (in which case,
errno
is set appropriately).
ERRORS
EBADF
oldfd
isn't an open file descriptor, or
newfd
is out of the allowed range for file descriptors.
EBUSY
(Linux only) This may be returned by
R dup2 ()
during a race condition with
open(2)
and
R dup ().
EINTR
The
R dup2 ()
call was interrupted by a signal.
EMFILE
The process already has the maximum number of file
descriptors open and tried to open a new one.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
The error returned by
R dup2 ()
is different from that returned by
R fcntl( ..., F_DUPFD , ... )
when
newfd
is out of range.
On some systems
R dup2 ()
also sometimes returns
EINVAL
like
R F_DUPFD .
If
newfd
was open, any errors that would have been reported at
close(2)
time, are lost.
A careful programmer will not use
R dup2 ()
without closing
newfd
first.
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