redhat man page for dup2

DUP(2)							     Linux Programmer's Manual							    DUP(2)

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  successful  return of dup or dup2, 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
       other.

       The two descriptors do not share the close-on-exec flag, however.

       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 appropriately).

ERRORS
       EBADF  oldfd isn't an open file descriptor, or newfd is out of the allowed range for file descriptors.

       EMFILE The process already has the maximum number of file descriptors open and tried to open a new one.

WARNING
       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.

CONFORMING TO
       SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents additional EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.

SEE ALSO
       fcntl(2), open(2), close(2)

Linux 1.1.46							    1994-08-21								    DUP(2)