Os.FdFile descriptors operations.
unix_buffer_size is the value of the OCaml runtime system buffer size for I/O operations.
apply ~close fd f calls f fd and ensure close fd is is called whenever the function returns. Any Unix.Unix_error raised by close fd is ignored.
copy ~buf src ~dst reads src and writes it to dst using buf as a buffer; if unspecified a buffer of length unix_buffer_size is created for the call. Raise Unix.Unix_error if that happens
to_string fd reads fd to a string. Raises Unix.Unix_error in case of error.