What does windows error 0 "ERROR_SUCCESS" mean?

I've written a python program which read a stdout of another process by the pipe redirection. However, the program sucks in this line:

print "[Input Thread] ", self.inputPipe.readline(ii)

The error is IOError: [Errno 0] Error

I found the explanatio开发者_高级运维n of windows errno 0. It makes confused because it defined as:

The operation completed successfully.

Why does an operation completed successfully lead to an error?

The name can trick you but ERROR_SUCCESS actually means there was no error.
From https://msdn.microsoft.com/en-us/library/windows/desktop/ms681382.aspx:

ERROR_SUCCESS

0 (0x0)
The operation completed successfully.

I know this is kind of old but having spent a fair amount of time trying to find a complete answer without success. So I figured I'd share what I've figured out.

The complete answer of how this happens, is when the pInvoke method you called "fails" but not because of an error.

huh you think

For example lets say you need to unhook a windows hook, but it gets called twice due to a bit of spaghetti or a paranoid level of defensive programming in your object architecture.

// hook assigned earlier

// now we call our clean up code
if (NativeMethods.UnhookWindowsHookEx(HookHandle) == 0)
{
    // method succeeds normally so we do not get here
    Log.ReportWin32Error("Error removing hook", Marshal.GetLastWin32Error());
}

// other code runs, but the hook is never reattached, 
// due to paranoid defensive program you call your clean up code twice
if (NativeMethods.UnhookWindowsHookEx(HookHandle) == 0)
{
    // pInvoke method failed (return zero) because there was no hook to remove
    // however there was no error, the hook was already gone thus ERROR_SUCCESS (0)
    // is our last error
    Log.ReportWin32Error("Error removing hook", Marshal.GetLastWin32Error());
}

The windows API can be tricky. Most likely, the error number was not properly retrieved by the second program you mentioned. It was either overwritten or not pulled in at all; so it defaulted to 0.

You didn't say what the other program was; but for example, in .net, it is easy to omit the 'SetLastError' flag when declaring your external calls.

[DllImport('kernel32.dll', SetLastError = true)]

https://www.medo64.com/2013/03/error-the-operation-completed-successfully/