DUP(2)                               BSD                                DUP(2)



NAME
     dup, dup2 - duplicate a descriptor

SYNOPSIS
     newd = dup(oldd)
     int newd, oldd;

     dup2(oldd, newd)
     int oldd, newd;

DESCRIPTION
     dup duplicates an existing object descriptor.  The argument oldd 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, newd, is the
     lowest-numbered descriptor not currently in use by the process.

     The object referenced by the descriptor does not distinguish between
     references using 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, you must
     obtain a different object reference to the file by issuing an additional
     open(2) call.  The close-on-exec flag on the new file descriptor is
     unset.

     In the second form of the call, the value of the newd desired is
     specified. If this descriptor is already in use, the descriptor is
     deallocated first, as if a close(2) call had been done first.

ERRORS
     dup and dup2 fail if either of the following is true:

     [EBADF]    oldd or newd is not a valid active descriptor.

     [EMFILE]   Too many descriptors are active.

SEE ALSO
     accept(2), open(2), close(2), fcntl(2), pipe(2), socket(2),
     socketpair(2), getdtablesize(2)

DIAGNOSTICS
     A successful call to either dup or dup2 returns 0.  A failed call returns
     -1 and sets errno to indicate the error.