[racket] How to invoke a blocking foreign function without blocking whole racket?

Hi,

 I try to wrap inotify API. After adding a watcher, I try to read events
from file descriptor. But the racket thread is blocking until the read
operation is done. Here is the definition of read.

(define _read (get-ffi-obj "read" libc (_fun #:async-apply (lambda (f) (f))
                                             #:save-errno 'posix
                                            _fd_t
                                            (output : (_bytes o size))
                                            (size : _uint32)
                                            -> (r : _int32)
                                            -> (values r saved-errno
output))))

I am confused by the term callback. Is the callback called in the racket
thread or another OS thread?

Thanks,
Haiwei
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