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This note corrects an error in the description of the Resource Manager routine
OpenRFPerm found in Inside Macintosh Volume IV
[Apr 01 1988]
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Introduction
On page IV-17 in the Resource Manager chapter of Inside Macintosh Volume
IV it states,
"OpenRFPerm, like OpenResFile, will not open the specified
file twice; it simply returns the reference number already assigned to the
file. In other words, OpenRFPerm cannot be used to open a second
access path to a resource file..."
This statement is incorrect. OpenRFPerm
behaves exactly as described if an application attempts to open a second access
path, with write access, to a resource file without
MultiFinder. With MultiFinder, however, if the second attempt comes from a
different application than the one that originally opened the file
OpenRFPerm will not return the reference number already
assigned to the file, instead it will return -1 and ResError will be
set to opWrErr (-49). In fact in similar circumstances with
MultiFinder, OpenResFile behaves the same way, returning -1 and
setting ResError to -49.
Note, however, that with or without MultiFinder, OpenRFPerm
will create multiple, unique, read-only access paths to a resource file.
Using this feature is not safe and should be avoided since if a resource file
is opened twice, once with read-write permission and once with read-only
permission then two copies of the resource map will exist in memory. If one of
the resources in the file is changed and written to disk then the two maps will
become inconsistent and the second access path will see what it thinks is a
trashed resource file.
If you must use this technique for read-only access, only call
OpenRFPerm when your application is ready to read information from the
file and close the file immediately afterwards. Otherwise you risk having the
resource file change out from under your application.
Back to top References
The Resource Manager
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