@feld @j @mint Yeah, that's why it's got to be lower-level; you don't want to put something semantically ugly in the higher level but you end up with a read giving you an immutable linked list of bytes instead of repeatedly refilling the same preallocated, aligned, contiguous region of memory. To do it without burning CPU, you can't allocate anything: it has to be loop{syscall; conditional break; syscall; conditional break;}. So the buffer can't cross into the user-level of the runtime unless you have an exact GC and you somehow have an internal allocator that can recognize that you're doing that. The former exists and I've never seen the latter, but you can tell it to copy one FD to the other without breaking the language and you can do that with a static buffer at a lower level where the semantics aren't expected to hold.