#include <unistd.h> int
dup (int oldd); int
dup2 (int oldd int newd);
DESCRIPTION
The
dup ();
system call
duplicates an existing object descriptor and returns its value to
the calling process
Fa ( newd
=
dup (oldd ) .);
The argument
Fa 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
Fa oldd
and
Fa newd
in any way.
Thus if
Fa newd
and
Fa 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
Fa newd
is specified.
If this descriptor is already in use and
Fa oldd
Fa newd ,
the descriptor is first deallocated as if the
close(2)
system call had been used.
If
Fa oldd
is not a valid descriptor, then
Fa newd
is not closed.
If
Fa oldd
==
Fa newd
and
Fa oldd
is a valid descriptor, then
dup2 ();
is successful, and does nothing.
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
The
dup ();
and
dup2 ();
system calls fail if:
Bq Er EBADF
The
Fa oldd
or
Fa newd
argument
is not a valid active descriptor
