Reading from Named Pipes

Both the server and the client processes read data from a pipe by calling DosRead. The server reads from the handle that was returned when it created the pipe with DosCreateNPipe, and the client reads from the handle that was returned to it by DosOpen.

When a pipe is created, the PipeMode parameter is used to specify both the pipe type and the read mode for the server end of the pipe:

• A byte pipe can be read-only in byte-read mode. (DosCreateNPipe and DosSetNPHState return ERROR_INVALID_PARAMETER if message-read mode is specified for a byte pipe.) In byte-read mode, all currently available data is returned, up to the buffer size specified by DosRead.

• A message pipe can be read in either byte-read mode or message-read mode, as follows:

  • When a message pipe is read in byte-read mode, the message headers are skipped, and the pipe is read as if it were a byte pipe.

  • When a message pipe is read in message-read mode, each message is read either in its entirety, or not at all, depending on the size of the message and the buffer length:

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    If the buffer length that was specified for DosRead is larger than the next available message, then only that message is read, and the Bytes-Read parameter indicates the size of the message.
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    If the buffer length for DosRead is smaller than the next available message, DosRead returns the number of bytes requested and ERROR_MORE_DATA. Subsequent calls to DosRead are blocked until the rest of the message can be transferred. (DosPeekNPipe can be used to find out how many bytes are left in the message.)

    The PipeMode parameter of DosCreateNPipe also specifies the blocking mode for the server end of the pipe:

    • If nonblocking mode was specified, DosRead returns immediately with 0 in the Bytes-Read parameter if no data is available.

    • If blocking mode was specified, DosRead blocks until data is available; the only time it will return with 0 in the Bytes-Read parameter is if it reaches end-of-file.

    DosRead works the same for both ends of the pipe. However, the read mode and blocking mode are not necessarily the same for the client end of the pipe as they are for the server end, because DosOpen always opens the CLIENT end in byte-read mode and blocking mode.

    The read mode and blocking mode for either end of the pipe can be changed by calling DosSetNPHState. The pipe type, however, is always the same for both the server and client ends of the pipe, and it cannot be changed.

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