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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
     The dup() system call duplicates an existing object descriptor and
     returns its value to the calling process (newd = dup(oldd)).  The argu-
     ment oldd is a small non-negative integer index in	the per-process
     descriptor	table.	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) system 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 the close(2) system 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.

     The related cap_new(2) system call	allows file descriptors	to be dupli-
     cated with	restrictions on	their use.

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
     The dup() system call fails if:

     [EBADF]		The oldd argument is not a valid active	descriptor

     [EMFILE]		Too many descriptors are active.
     The dup2()	system call fails if:

     [EBADF]		The oldd argument is not a valid active	descriptor or
			the newd argument is negative or exceeds the maximum
			allowable descriptor number

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

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

HISTORY
     The dup() and dup2() functions appeared in	Version	7 AT&T UNIX.

FreeBSD	10.1			April 21, 2013			  FreeBSD 10.1

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

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