Once an I/O resource has been opened, a different namespace
comes into play. The open() returns an integer
referred to as a file descriptor (FD), which is used
to direct all further I/O requests to that resource manager.
Unlike the pathname space, the file descriptor namespace is
completely local to each process. The resource manager uses
the combination of a SCOID (server connection ID) and FD (file
descriptor/connection ID) to identify the control structure
associated with the previous open() call.
This structure is referred to as an open control block
(OCB) and is contained within the resource manager.
The following diagram shows an I/O manager taking some
SCOID, FD pairs and mapping them to OCBs:
Figure 1The SCOID and FD map to an OCB of an I/O Manager.
