#include <unistd.h> int
dup (int oldd); int
dup2 (int oldd int newd);
duplicates an existing object descriptor and returns its value to
the calling process
Fa ( newd
dup (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
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
in any way.
are duplicate references to an open
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
The close-on-exec flag on the new file descriptor is unset.
the value of the new descriptor
If this descriptor is already in use and
Fa newd ,
the descriptor is first deallocated as if the
system call had been used.
is not a valid descriptor, then
is not closed.
is a valid descriptor, then
is successful, and does nothing.
The value -1 is returned if an error occurs in either call.
The external variable
indicates the cause of the error.
system calls fail if:
Bq Er EBADF
is not a valid active descriptor