dup man page on 4.4BSD

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DUP(2)			    BSD System Calls Manual			DUP(2)

NAME
     dup, dup2 — duplicate an existing file descriptor

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, the descriptor is first deallocated as if a
     close(2) call had been done first.

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

STANDARDS
     Dup() and dup2() are expected to conform to IEEE Std 1003.1-1988
     (“POSIX”).

4th Berkeley Distribution	 June 4, 1993	     4th Berkeley Distribution
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