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ERROR_PIPE_CONNECTED
ERROR_PIPE_CONNECTED can be a successful connection condition for a named-pipe server. It is most important when handling the return value of ConnectNamedPipe.
A race that already produced a connection
A client can connect after the server creates a pipe instance but before the server calls ConnectNamedPipe. In that case, ConnectNamedPipe returns FALSE and GetLastError() returns ERROR_PIPE_CONNECTED, even though the server and client are connected correctly.
Correct server behavior
For this specific ConnectNamedPipe case, treat the pipe as connected and continue to the read or request-processing state. In an overlapped implementation, arrange for the connection state to be processed exactly once, just as if a normal completion notification had arrived.
Common mistake
Closing the handle because ConnectNamedPipe returned FALSE drops a valid client connection. The return value must always be interpreted together with GetLastError().
See Microsoft documentation for ConnectNamedPipe.
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