From Linux Man Pages
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 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.
dup() uses the lowest-numbered unused descriptor for the new descriptor.
dup2() makes newfd be the copy of oldfd, closing newfd first if necessary.
RETURN VALUE
dup() and dup2() return the new descriptor, or -1 if an error occurred (in which case, errno is set appropri-
ately).
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 condition with open() 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.
WARNINGS
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() time, are lost. A careful programmer will
not use dup2() without closing newfd first.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
RELATED
close(2), fcntl(2), open(2)
CATEGORY