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

NAME
     dup, dup2 -- 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);

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
     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 and oldd != newd, the	descriptor is first
     deallocated as if a close(2) call had been	used.  If oldd is not a	valid
     descriptor, then newd is not closed.  If oldd == newd and oldd is a valid
     descriptor, then dup2() is	successful, and	does nothing.

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),	close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
     socket(2),	socketpair(2)

STANDARDS
     The dup() and dup2() function calls are expected to conform to ISO/IEC
     9945-1:1990 (``POSIX.1'').

HISTORY
     A dup() and a dup2() function call	appeared in Version 7 AT&T UNIX.

FreeBSD	9.2			 June 4, 1993			   FreeBSD 9.2

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

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