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manpages-dev_2.62-1_all 
NAME
dup, dup2 - duplicate a file descriptor
SYNOPSIS
#include <unistd.h>
int dup(int oldfd);
int dup2(int oldfd, int newfd);
DESCRIPTION
dup() and dup2() create a copy of the file descriptor oldfd.
After a successful return from dup() or 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 sta‐
tus 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.
dup() uses the lowest-numbered unused descriptor for the new descrip‐
tor.
dup2() makes newfd be the copy of oldfd, closing newfd first if neces‐
sary.
dup() and 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 dup2() during a race condi‐
tion with open(2) and dup().
EINTR The 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.
SVr4, 4.3BSD, POSIX.1-2001.
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() without clos‐
ing newfd first.
close(2), fcntl(2), open(2)