with-open-file

(with-open-file (stream filename options...) body...)

Opens the file named by filename, binds the open stream to stream, evaluates the body forms with that binding, and closes the file afterwards, returning the value of the last body form. On the interpreter and the JVM the expansion wraps the body in unwind-protect, so the file is closed on every exit (normal return, an error signaled in the body, or a return/return-from); on the WASM backends, where unwind-protect does not compile, it is closed on normal exit only. The supported options are :direction -- the literal keyword :input (the default) or :output -- and :element-type -- the literal 'character (the default, a text stream) or '(unsigned-byte 8) (a binary stream for read-byte/write-byte). Both must be literal so the compilers can pick the file mode statically. It expands into a plain open/close pair, so no special stream type is involved.

Because it touches the filesystem, with-open-file is shown here statically rather than as a runnable example:

(with-open-file (s "out.txt" :direction :output)
  (write-line "hello" s))
(with-open-file (s "out.txt" :direction :input)
  (read-line s)) ; => "hello"
(with-open-file (s "out.bin" :direction :output :element-type '(unsigned-byte 8))
  (write-byte 255 s)) ; => 255