Reset GetLastError value

Delphi Xe.

in delphi help: "...Calling this function usually resets the operating system error state..."

How to res开发者_如何学Pythonet a current error on 0? I.e. that GetLastError=0

Example:

Try
// There is an error
except
showmessage(inttostr(getlasterror)); // Ok, getlasterror<>0
end;
....
// no errors
....
// How to reset a current error on 0?
showmessage(inttostr(getlasterror)); // Again will be <> 0

You should only be calling GetLastError when there has actually been an error. Some Windows API functions will reset the error to 0 on success, some won't. Either way, you should only interrogate the error state when you need to know the most recent error.

Note that there is a SetLastError method as well, but that doesn't help you; if you set the last error to 0, then of course GetLastError will return 0.

It's actually a Win32 call (not Delphi per se).

You can clear it with "SetLastError ()".

Here's the MSDN documentation:

http://msdn.microsoft.com/en-us/library/ms679360%28v=vs.85%29.aspx

This is an example of low quality documentation. GetLastError WinAPI function will retain its return value until next call of SetLastError so repeatedly invoking will have no effect.

SetLastError(42);
for I := 1 to 100 do
  Assert(GetLastError() = 42); // all of those assertions evaluates to True

Also, in Delphi documentation GetLastError has been misplaced into exception handling routines; this is wrong too, these error handling mechanisms are unrelated to each other.

On that silly "usually" word in the reference: It happens because function used to output GetLastError return value invokes SetLastError. Eg:

  SetLastError(42);
  OutputDebugString(PChar(Format('GetLastError() = %d', [GetLastError()]))); // 42
  OutputDebugString(PChar(Format('GetLastError() = %d', [GetLastError()]))); // 0! (ERROR_SUCCESS set by the previous OutputDebugString call)