DUP(2)								System Calls Manual							    DUP(2)

NAME

       dup, dup2 - duplicate a descriptor

SYNOPSIS

       #include <unistd.h>

       int dup(int oldd)
       int dup2(int oldd, int newd)

DESCRIPTION

       Dup  duplicates	an existing descriptor.  The argument oldd is a small non-negative integer index in the per-process descriptor table.  The
       value must be less than OPEN_MAX, the size of the table.  The new descriptor returned by the call, let's name it newd, is the  lowest  num-
       bered descriptor that is not currently in use by the process.

       The  object referenced by the descriptor does not distinguish between references using 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  the	second	form  of  the call, the value of newd desired is specified.  If this descriptor is already in use, the descriptor is first
       deallocated as if a close(2) call had been done first.  Newd is not closed if it equals oldd.

RETURN VALUE

       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.

NOTES

       Dup and dup2 are now implemented using the F_DUPFD function of fcntl(2), although the old system call interfaces still exist to support old
       programs.

SEE ALSO

       open(2), close(2), fcntl(2), pipe(2).

4th Berkeley Distribution					   May 13, 1986 							    DUP(2)