This subsection describes a conceptual model of the UNIX pipe stream.

In the C language, three file descriptors are made available to processes: stdin, stdout, and stderr. The stdin file descriptor is used to provide input to the process, while stdout and stderr are for output and error output respectively. The Smalltalk interface to these pipe handles is either an instance of UNIXWritePipeStream or UNIXReadPipeStream.

Both the UNIXWritePipeStream and UNIXReadPipeStream classes respond to a subset of the stream protocol. The pipe stream interface permits modification of the position in the stream. This means that a UNIXReadPipeStream can be reset and re-read; similarly, a UNIXWritePipeStream can be reset and the same input flushed a second time.

Instances of UNIXWritePipeStream and UNIXReadPipeStreams should never be created directly. They should be requested when the process is created through the execute:input:output:error:environment: message supported by UNIXProcess.