dup, dup2 - duplicate a file descriptor
int dup(int oldfd));
int dup2(int oldfd, int newfd));
dup and dup2 create a copy of the file descriptor oldfd.
The old and new descriptors may be used interchangeably.
They share locks, file position pointers and flags; for
example, if the file position is modified by using lseek on
one of the descriptors, the position is also changed for the
The two descriptors do not share the close-on-exec flag,
dup uses the lowest-numbered unused descriptor for the new
dup2 makes newfd be the copy of oldfd, closing newfd first
dup and dup2 return the new descriptor, or -1 if an error
occurred (in which case, errno is set appropriately).
oldfd isn't an open file descriptor, or newfd is out of
the allowed range for file descriptors.
The process already has the maximum number of file
descriptors open and tried to open a new one.
The error returned by dup2 is different to that returned by
fcntl(...,F_DUPFD,...) when newfd is out of range. On some
systems dup2 also sometimes returns EINVAL like F_DUPFD.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents addi-
tional EINTR and ENOLINK error conditions. POSIX.1 adds
fcntl (2), open (2), close (2).