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Unix Version 7 - man page for dup (v7 section 2)

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DUP(2)											   DUP(2)

       dup, dup2 - duplicate an open file descriptor

       int fildes;

       dup2(fildes, fildes2)
       int fildes, fildes2;

       Given  a file descriptor returned from an open, pipe, or creat call, dup allocates another
       file descriptor synonymous with the original.  The new file descriptor is returned.

       In the second form of the call, fildes is a file descriptor referring to an open file, and
       fildes2 is a non-negative integer less than the maximum value allowed for file descriptors
       (approximately 19).  Dup2 causes fildes2 to refer to the same file as fildes.  If  fildes2
       already referred to an open file, it is closed first.

       creat(2), open(2), close(2), pipe(2)

       The  value  -1 is returned if: the given file descriptor is invalid; there are already too
       many open files.

       (dup = 41.)
       (file descriptor in r0)
       (new file descriptor in r1)
       sys dup
       (file descriptor in r0)

       The dup2 entry is implemented by adding 0100 to fildes.

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