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DUP(2)			       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 argument 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  op-
     tions  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 and close-
     on-fork  flags  on  the new file descriptor are unset.  The resolve-beneath
     flag on the new file descriptor is set to the same state as on the old file
     descriptor.

     In dup2(), the value of the new descriptor newd is specified.  If this  de-
     scriptor  is already in use and oldd != newd, the descriptor is first deal-
     located 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.

RETURN VALUES
     These calls return the new file descriptor  if  successful;  otherwise  the
     value -1 is returned and the external variable errno is set to indicate 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 al-
			lowable descriptor number

SEE ALSO
     accept(2),  close(2),   fcntl(2),	 getdtablesize(2),   open(2),	pipe(2),
     socket(2), socketpair(2), dup3(3)

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

HISTORY
     The dup() function appeared in Version 3 AT&T UNIX.   The	dup2()	function
     appeared in Version 7 AT&T UNIX.

FreeBSD ports 15.quarterly	December 1, 2017			  DUP(2)

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