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

NAME
     dup, dup2,	dup3 --	duplicate an existing file descriptor

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int	newd);

     int
     dup3(int oldd, int	newd, int flags);

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(3).	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) 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.	 If newd and oldd are the same,	the
     call has no effect.

     dup3() behaves exactly like dup2()	only it	allows extra flags to be set
     on	the returned file descriptor.  The following flags are valid:

	   O_CLOEXEC   Set the "close-on-exec" property.

	   O_NONBLOCK  Sets non-blocking I/O.

	   O_NOSIGPIPE
		       Return EPIPE instead of raising SIGPIPE.

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
     All three functions may fail if:

     [EBADF]		oldd is	not a valid active descriptor or newd is not
			in the range of	valid file descriptors.

     The dup() function	may also fail if:

     [EMFILE]		Too many descriptors are active.

     The dup3()	function will also fail	if:

     [EINVAL]		flags is other than O_NONBLOCK or O_CLOEXEC.

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

STANDARDS
     The dup() and dup2() functions conform to ISO/IEC 9945-1:1990
     ("POSIX.1").

HISTORY
     The dup3()	function is inspired from Linux	and appeared in	NetBSD 6.0.

BSD			       January 23, 2012				   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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