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DUP(2V)                                                                DUP(2V)



NAME
       dup, dup2 - duplicate a descriptor

SYNOPSIS
       int dup(fd)
       int fd;

       int dup2(fd1, fd2)
       int fd1, fd2;

DESCRIPTION
       dup()  duplicates  an existing object descriptor.  The argument fd is a
       small non-negative integer index in the per-process  descriptor  table.
       The value must be less than the size of the table, which is returned by
       getdtablesize(2).  The new descriptor returned by the call is the  low-
       est numbered descriptor that is not currently in use by the process.

       With dup2(), fd2 specifies the desired value of the new descriptor.  If
       descriptor fd2 is already in use, it is first deallocated as if it were
       closed by close(2V).

       The new descriptor has the following in common with the original:

       o  It refers to the same object that the old descriptor referred to.

       o  It uses the same seek pointer as the old descriptor.  (that is, both
          file descriptors share one seek pointer).

       o  It has the same access mode (read, write or read/write) as  the  old
          descriptor.

       Thus if fd2 and fd1 are duplicate references to an open file, read(2V),
       write(2V), and lseek(2V) calls all move a single seek pointer into  the
       file,  and  append  mode, non-blocking I/O and asynchronous I/O options
       are shared between the references.  If a separate seek pointer into the
       file  is  desired,  a  different  object  reference to the file must be
       obtained by issuing an additional  open(2V)  call.   The  close-on-exec
       flag on the new file descriptor is unset.

       The  new file descriptor is set to remain open across exec system calls
       (see fcntl(2V).

RETURN VALUES
       dup() and dup2() return a new descriptor on success.  On failure,  they
       return -1 and set errno to indicate the error.

ERRORS
       EBADF          fd1 or fd2 is not a valid active descriptor.

       EMFILE         Too many descriptors are active.

SEE ALSO
       accept(2), close(2V), fcntl(2V), getdtablesize(2), lseek(2V), open(2V),
       pipe(2V), read(2V), socket(2), socketpair(2), write(2V)



                                21 January 1990                        DUP(2V)