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NetBSD 6.1.5 - man page for dup2 (netbsd section 2)

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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 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.  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
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