OpenDarwin 7.2.1 - man page for dup2 (opendarwin section 2)

DUP(2)				     BSD System Calls Manual				   DUP(2)

NAME
     dup, dup2 -- duplicate an existing file descriptor

SYNOPSIS
     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int newd);

DESCRIPTION
     Dup() 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 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 lowest num-
     bered 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) 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, the descriptor is first deallocated as if a close(2) call had been done first.

RETURN VALUES
     The value -1 is returned if an error occurs in either call.  The external variable errno
     indicates the cause of the error.

ERRORS
     Dup() and dup2() fail if:

     [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)

STANDARDS
     Dup() and dup2() are expected to conform to IEEE Std 1003.1-1988 (``POSIX.1'').

4th Berkeley Distribution		   June 4, 1993 		4th Berkeley Distribution