with-output-to-string

(with-output-to-string (stream) body...)

Binds stream to a string output stream, evaluates the body forms, and returns everything written to the stream as a string. princ, prin1, print, terpri, write-line and write-string accept the stream as their optional stream argument, and format accepts it as the destination; each call appends to the stream. Works in all three backends.