You could create a Scheme port from the file descriptor and then use the normal I/O operations on that. Here is some (untested) code to illustrate how this can be done:

(define direction-in 1) (define direction-out 2) (define direction-inout 3)

(define (fd->port fd mode) (##open-predefined mode '(fd-port) fd))

(define fd (get-fd-using-ffi!))

(define my-port (fd->port fd direction-inout))

... use my-port like a normal port ...

Thanks, this solves one part of the problem. But the scheme code must not read or write any byte from/to my-port. And I don't see a way to implement (if (foreign-function-tells-io-would-block) (suspend-me-until-io-will-not-block)) without reading or writing. Regards Hermann P.S.: Is ##open-predefined considered stable?