#include <unistd.h>int dup(int oldfd);int dup2(int oldfd, int newfd);#define _GNU_SOURCE#include <unistd.h>int dup3(int oldfd, int newfd, int flags);
DESCRIPTION
These system calls create a copy of the file descriptor
oldfd.
dup()
uses the lowest-numbered unused descriptor for the new descriptor.
dup2()
makes newfd be the copy of oldfd, closing newfd
first if necessary, but note the following:
*
If
oldfd
is not a valid file descriptor, then the call fails, and
newfd
is not closed.
*
If
oldfd
is a valid file descriptor, and
newfd
has the same value as
oldfd,
then
dup2()
does nothing, and returns
newfd.
After a successful return from one of these system calls,
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.
dup3()
is the same as
dup2(),
except that:
*
The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
O_CLOEXEC
in
flags.
See the description of the same flag in
open(2)
for reasons why this may be useful.
*
If
oldfd
equals
newfd,
then
dup3()
fails with the error
EINVAL.
RETURN VALUE
On success, these system calls
return the new descriptor.
On error, -1 is returned, and
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
dup2()
or
dup3()
during a race condition with
open(2)
and
dup().
EINTR
The
dup2()
or
dup3()
call was interrupted by a signal; see
signal(7).
EINVAL
(dup3())
flags
contain an invalid value.
Or,
oldfd
was equal to
newfd.
EMFILE
The process already has the maximum number of file
descriptors open and tried to open a new one.
VERSIONS
dup3()
was added to Linux in version 2.6.27;
glibc support is available starting with
version 2.9.
CONFORMING TO
dup(),
dup2():
SVr4, 4.3BSD, POSIX.1-2001.
dup3()
is Linux-specific.
NOTES
The error returned by
dup2()
is different from that returned by
fcntl(..., F_DUPFD, ...)
when
newfd
is out of range.
On some systems
dup2()
also sometimes returns
EINVAL
like
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
dup2()
or
dup3()
without closing
newfd
first.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
