| Q What is the correct way to access a File
Control Block for an open file?
A The correct way to access the information
from the FCB is using PBGetFCBInfo. In some cases, it
may also be correct to use the File System Manager (FSM) FCB
accessors. Under no circumstances should you access the file control
block table or file control blocks (FCBs) directly. You've been
warned many times about this in the past: - Inside Macintosh II, page 127, "Warning: The size and structure of a file control block may be different in future versions of Macintosh system software."
- Inside Macintosh IV, page 181, "Warning: The size and structure of a file control block may be different in future versions of Macintosh system software."
- Inside Macintosh V, page 386, "Do not directly examine or manipulate system data structures, such as file control blocks (FCB) or volume control blocks (VCB), in memory. Use File Manager calls to access FCB and VCB information."
- Inside Macintosh: Files, page 2-81, "Note: The size and structure of a file control block may be different in future versions of Macintosh system software. To be safe, you should get information from the FCB allocated for an open file by calling the File Manager function
PBGetFCBInfo."
Now, we really mean it. There is really only one good reason to access a FCB directly and
that is: your code is the file system that owns the volume the open
file is on. Applications should always use PBGetFCBInfo to get
information from the FCB for an open file. The few fields in a FCB
not returned by PBGetFCBInfo are either file system-
specific, or they are reserved for the File Manager's own use. (For
example, some fields in an FCB are used for different purposes by the
Macintosh file system code for accessing HFS or HFS Plus volumes than
what they are used for by the File Exchange file system code for
accessing DOS or ProDOS volumes.) In addition, some network file
systems (for example, AppleShare) use PBGetFCBInfo to
update the information they keep in the FCB and return in the
FCBPBRec, so accessing the FCB directly may give you
stale data. However, we know there are reasons for patches to file system
traps or debugging tools (such as MacsBug's FILE dcmd) to look
directly at a file control block. So in the interest of allowing
useful things to keep working, and to allow Apple to provide future
enhancements to the File Manager, we will soon require code that
accesses FCBs to use a single set of accessor functions to get to
FCBs. The accessor functions have been public since 1994 as part of
the File System Manager (FSM) and the File System Manager has been
part of Mac OS since System 7.5. You can check for the presence of
FSM with Gestalt by using the gestaltFSAttr selector and checking for the
gestaltHasFileSystemManager bit. |