Command Section
DUP(2)                    FreeBSD System Calls Manual                   DUP(2)

     dup, dup2 - duplicate an existing file descriptor

     Standard C Library (libc, -lc)

     #include <unistd.h>

     dup(int oldd);

     dup2(int oldd, int newd);

     The dup() system call duplicates an existing object descriptor and
     returns its value to the calling process (newd = dup(oldd)).  The
     argument oldd is a small non-negative integer index in the per-process
     descriptor table.  The new descriptor returned by the call is the lowest
     numbered descriptor currently not in use by the process.

     The object referenced by the descriptor does not distinguish between oldd
     and newd in any way.  Thus if newd and oldd are duplicate references to
     an open file, read(2), write(2) and lseek(2) calls all move a single
     pointer into the file, and append mode, non-blocking I/O and asynchronous
     I/O options are shared between the references.  If a separate pointer
     into the file is desired, a different object reference to the file must
     be obtained by issuing an additional open(2) system call.  The close-on-
     exec flag on the new file descriptor is unset.

     In dup2(), the value of the new descriptor newd is specified.  If this
     descriptor is already in use and oldd != newd, the descriptor is first
     deallocated as if the close(2) system call had been used.  If oldd is not
     a valid descriptor, then newd is not closed.  If oldd == newd and oldd is
     a valid descriptor, then dup2() is successful, and does nothing.

     These calls return the new file descriptor if successful; otherwise the
     value -1 is returned and the external variable errno is set to indicate
     the cause of the error.

     The dup() system call fails if:

     [EBADF]            The oldd argument is not a valid active descriptor

     [EMFILE]           Too many descriptors are active.

     The dup2() system call fails if:

     [EBADF]            The oldd argument is not a valid active descriptor or
                        the newd argument is negative or exceeds the maximum
                        allowable descriptor number

     accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
     socket(2), socketpair(2), dup3(3)

     The dup() and dup2() system calls are expected to conform to IEEE Std
     1003.1-1990 (``POSIX.1'').

     The dup() and dup2() functions appeared in Version 7 AT&T UNIX.

FreeBSD 11.1-RELEASE-p4          June 1, 2013          FreeBSD 11.1-RELEASE-p4
Command Section