Re: [Boost-users] boost::filesystem::exists() & GetfileAttributesA

From: Beman Dawes (bdawes_at_[hidden])
Date: 2004-03-29 09:12:55

At 06:30 AM 3/25/2004, John Maddock wrote:
 >>  >We had a similar problem in one of our apps with GetFileAttributesA.
 >The
 >>  >solution was to switch to FindFirstFile, FindClose, giving raise to 
the
 >>  >suspicion that some implementations of GetFileAttributes are broken.
 >>
 >> That would be a truly horrible bug. GetFileAttributes() isn't just some
 >> minor, rarely used function. It is critical to the correct operation of 
a
 >> number of Boost.Filesystem functions, and I'd guess many tens of
 >thousands
 >> of other applications. It is hard to imagine a Windows system staying
 >> stable if GetFileAttributes() failed very often.
 >
 >There is some information in MSDN on this: First off I found an "Under 
the
 >Hood" article from the September 2000 issue of MSDN Magazine, where
 >GetFileAttributes was periodically failing due to an access permissions
 >problem.  The secret was to determine the value of GetLastError after the
 >call

Makes sense.

 > (apparently you can get this without actually calling GetLastError by
 >examining the DWORD at offset 0x34 in the thread environment block 
pointed
 >to by FS:[18h]).

Unless that is a published interface unlikely to change, I'd rather call 
GetLastError(). An offset also might change in a 64-bit Windows 
environment, which we are likely to be having to deal with soon.

 >So if GetFileAttributes can fail for reasons other than the file not
 >existing, then the return value of GetLastError should also be checked to
 >verify that the reason for the failure really was that the file does not
 >exist: Looks like error codes of ERROR_FILE_NOT_FOUND or
 >ERROR_PATH_NOT_FOUND would be returned in this case.

That sounds right to me.

GetFileAttributes() is also used by is_directory(), and 
GetFileAttributesEx() is used a couple of places. I don't know if those 
usages also need to be changed. It may be the code is OK but the docs need 
to mention the case where attributes are unaccessible.

John, could you take a look at 
boost-root/libs/filesystem/src/operations_posix_windows.cpp and make any 
changes you think are necessary? I'm out-of-action for personal reasons for 
awhile, so can't do it myself.

Thanks,

--Beman