make-broadcast-stream

(make-broadcast-stream &rest streams)

An output stream that fans every write out to each component stream, in the order given. With no components it is a broadcast stream over an empty list instead: writes to it are dropped (the CL idiom for a null output stream), and the file queries answer for nothing -- file-length 0, file-position 0, file-string-length 1, stream-external-format :default. With components the same queries answer the last component.

A broadcast stream WITH components is a Gray stream, so it takes the whole output protocol: format, princ, prin1, print, write-string, write-char, terpri, fresh-line, write-line, force-output, finish-output, clear-output and close. fresh-line answers the last component, or nil with no components.