Blocking and Nonblocking Pipe Operations

The wait mode (blocking or nonblocking) of a pipe handle determines whether the ReadFile, WriteFile, and ConnectNamedPipe functions return immediately in situations that would otherwise require an indefinite wait. By default, all named pipe handles returned by the CreateNamedPipe or CreateFile function are created with blocking-wait mode enabled. Nonblocking-wait mode can be enabled for a server's pipe handle by specifying PIPE_NOWAIT in CreateNamedPipe. Both server and client processes can change a pipe handle's wait mode by specifying either PIPE_WAIT or PIPE_NOWAIT in a call to the SetNamedPipeHandleState function.

Note that nonblocking-wait mode is supported for compatibility with Microsoft® LAN Manager version 2.0, and it should not be used to achieve overlapped input and output (I/O) with named pipes. Overlapped I/O should be used instead because it enables time-consuming ReadFile, WriteFile, and ConnectNamedPipe operations to execute in the background after the function returns. For more information about overlapped I/O, see Synchronous and Overlapped Input and Output.

A ReadFile operation is affected by the wait mode of a pipe handle when the pipe is empty. With a blocking-wait handle, the operation is not completed successfully until data is available from a thread writing to the other end of the pipe. Using a nonblocking-wait handle, the function returns FALSE immediately, and the GetLastError function returns ERROR_NO_DATA.

A WriteFile operation is affected by a pipe handle's wait mode when there is insufficient space in the pipe's buffer to hold all the data. With a blocking-wait handle, the write operation cannot succeed until sufficient space is created in the buffer by a thread reading from the other end of the pipe. With a nonblocking-wait handle, the write operation returns TRUE immediately, without writing any bytes (for a message-type pipe) or after writing as many bytes as the buffer holds (for a byte-type pipe).

A ConnectNamedPipe operation is affected by the wait mode of a pipe handle when there is no client connected or waiting to connect to the pipe instance. With a blocking-wait handle, the connect operation does not succeed until a client process connects to the pipe instance by calling either the CreateFile or CallNamedPipe function. With a nonblocking-wait handle, the connect operation returns FALSE immediately, and the GetLastError function returns ERROR_PIPE_LISTENING.

The wait mode of a pipe handle does not affect read or write operations when using the TransactNamedPipe or CallNamedPipe function.