fd_send_recv: add fd_send_substring as for write in the ocaml std library

See https://github.com/ocaml/ocaml/blob/trunk/otherlibs/unix/unix.ml#L328-L332

Signed-off-by: Marcello Seri <[email protected]>

Author: Marcello Seri

lib/fd_send_recv.ml

@@ -19,6 +19,9 @@ let _ = Callback.register_exception "fd_send_recv.unix_error" (Unix_error (0))
 external send_fd : Unix.file_descr -> bytes -> int -> int -> Unix.msg_flag list -> Unix.file_descr -> int = "stub_unix_send_fd_bytecode" "stub_unix_send_fd"
 external recv_fd : Unix.file_descr -> bytes -> int -> int -> Unix.msg_flag list -> int * Unix.sockaddr * Unix.file_descr = "stub_unix_recv_fd"
 
+let send_fd_substring channel_fd buf ofs len flags fd_to_send =
+    send_fd channel_fd (Bytes.unsafe_of_string buf) ofs len flags fd_to_send
+
 let fd_of_int (x: int) : Unix.file_descr = Obj.magic x
 
 let int_of_fd (x: Unix.file_descr) : int = Obj.magic x

lib/fd_send_recv.mli

@@ -32,6 +32,10 @@ val recv_fd : Unix.file_descr -> bytes -> int -> int ->
     substring [buf] [ofs] [len] with [flags], returning the number of
     bytes read, the address of the peer and a file descriptor. *)
 
+val send_fd_substring : Unix.file_descr -> string -> int -> int ->
+  Unix.msg_flag list -> Unix.file_descr -> int
+(** Like [send_fd] but takes a string *)
+
 val int_of_fd : Unix.file_descr -> int
 (** [int_of_fd fd] returns the underlying unix integer file descriptor
     associated with OCaml Unix.file_descr [fd]. *)