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     dup, dup2 — duplicate an existing file descriptor


     Standard C Library (libc, -lc)


     #include <unistd.h>

     dup(int oldd);

     dup2(int oldd, int newd);


     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 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

     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 olddnewd, 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 value -1 is returned if an error occurs in either call.  The external
     variable errno indicates the cause of the error.


     The dup() and dup2() system calls fail if:

     [EBADF]            The oldd or newd argument is not a valid active

     [EMFILE]           Too many descriptors are active.


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


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


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