Creates a new fixup port. A fixup port acts as an output port that writes to an internal buffer. It also supports a stack of fixups—locations in the output buffer where additional data must be inserted later. The primary use case for fixups is length-prefixed data, where the length is not known in advance.

Returns #t if v is a fixup port created by open-fixup-port, #f otherwise.

Pushes a new fixup corresponding to the current location in the output buffer. If size is an integer, then size bytes are reserved for the fixup and its value must be exactly size bytes; otherwise the fixup’s value may be any size. (Sized fixups may have better performance than unsized fixups.)

Pops the current fixup and sets its value to the result of (fixup len), where len is the number of bytes (including subsequent fixup values) written to fp since the fixup was pushed.

Writes the buffered output and fixups to out. There must be no pending fixups on fp; otherwise an exception is raised.