[Ace-users] pread/pwrite implementation for ACE_HAS_WINNT4 performance

[daniel.schenk at schiller.ch]

Hi

in ace\OS_NS_unistd.cpp the ACE_WIN32 implementation of ACE_OS::pread
uses
1. ACE_OS_GUARD to synchronize access
2. gets the current file pointer, set it to the desired offset and
restores it after reading

Both should be unnecessary if ACE_HAS_WINNT4 is defined.
Because ::ReadFile is used with an OVERLAPPED structure. ::ReadFile
will set the offset internally, which is passed inside the OVERLAPPED
structure. Therefore there will be no need for synchronization by
ACE_OS_GUARD.
This will mean that the current implementation for  ACE_HAS_WINNT4
unnecessarily calls  SetFilePointer 3 times and locks, which will of
course slow down the performance.
The same will be true for ACE_OS::pwrite.

Please correct me, if I'm wrong. Otherwise I would appreciate if you
could change the code.

best regards

Daniel Schenk