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

From: John Maddock (john_at_[hidden])
Date: 2004-03-25 06:30:04

>  >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 (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]).

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.

John.