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

From: John Maddock (john_at_[hidden])
Date: 2004-03-30 10:29:59

>  > (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.

Absolutely, I mentioned that only because (in some debuggers apparently) it
let's you inspect the result of GetLastError in the debugger, even if your
code doesn't actually call it.

>  >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.

I've taken a quick look, and they all throw exceptions if the call fails,
that looks OK to me, it's just exists() that has to produce a true/false
result with no exceptions.

> 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.

OK, patch applied below, note the changes to the stat() code as well - that
API can also fail for reasons other than the file not existing.

John.

diff -u -r1.32 operations_posix_windows.cpp
--- operations_posix_windows.cpp        20 Mar 2004 18:24:48 -0000      1.32
+++ operations_posix_windows.cpp        30 Mar 2004 15:06:44 -0000
@@ -310,9 +310,23 @@
     {
 #   ifdef BOOST_POSIX
       struct stat path_stat;
-      return ::stat( ph.string().c_str(), &path_stat ) == 0;
+      if(::stat( ph.string().c_str(), &path_stat ) != 0)
+      {
+         if((errno == ENOENT) || (errno == ENOTDIR))
+            return false;  // stat failed because the path does not exist
+         // for any other error we assume the file does exist and fall
through,
+         // this may not be the best policy though...  (JM 20040330)
+      }
+      return true;
 #   else
-      return ::GetFileAttributesA( ph.string().c_str() ) != 0xFFFFFFFF;
+      if(::GetFileAttributesA( ph.string().c_str() ) == 0xFFFFFFFF)
+      {
+         if((::GetLastError() == ERROR_FILE_NOT_FOUND) || (::GetLastError()
== ERROR_PATH_NOT_FOUND))
+            return false; // GetFileAttributes failed because the path does
not exist
+         // for any other error we assume the file does exist and fall
through,
+         // this may not be the best policy though...  (JM 20040330)
+      }
+      return true;
 #   endif
     }