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DUP(2)			  FreeBSD System Calls Manual			DUP(2)

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

SYNOPSIS
     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int	newd);

     #include <fcntl.h>
     #include <unistd.h>

     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, it is first deallocated as if a close(2)
     call had been done	first.	When newd equals oldd, dup2() just returns
     without affecting the close-on-exec flag.

     In	dup3(),	both the value of the new descriptor and the close-on-exec
     flag on the new file descriptor are specified: newd specifies the value
     and the O_CLOEXEC bit in flags specifies the close-on-exec	flag.  Unlike
     dup2(), if	oldd and newd are equal	then dup3() fails.  Otherwise, if
     flags is zero then	dup3() is identical to a call to dup2().

RETURN VALUES
     Upon successful completion, the value of the new descriptor is returned.
     The value -1 is returned if an error occurs in either call.  The external
     variable errno indicates the cause	of the error.

ERRORS
     dup() will	fail if:

     [EBADF]		oldd is	not a valid active descriptor.

     [EMFILE]		Too many descriptors are active.

     dup2() and	dup3() will fail if:

     [EBADF]		oldd is	not a valid active descriptor or newd is nega-
			tive or	greater	than or	equal to the process's
			RLIMIT_NOFILE limit.

     [EBUSY]		A race condition with accept(2)	or open(2) has been
			detected.

     [EINTR]		An interrupt was received.

     [EIO]		An I/O error occurred while writing to the file	sys-
			tem.

     In	addition, dup3() will return the following error:

     [EINVAL]		oldd is	equal to newd or flags is invalid.

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

STANDARDS
     dup() and dup2() conform to IEEE Std 1003.1-2008 ("POSIX.1").  The	dup3()
     function is expected to conform to	a future revision of that standard.

HISTORY
     The dup() system call first appeared in Version 3 AT&T UNIX, dup2() in
     Version 7 AT&T UNIX, and dup3() in	OpenBSD	5.7.

FreeBSD	13.0			 June 25, 2018			  FreeBSD 13.0

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

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