Add notif and related SendZc docs (#299)

* Add notif that checks for IORING_CQE_F_NOTIF

* Add extra documentation to SendZc

---------

Co-authored-by: Benedek Thaler <benedek.thaler@wincent.co>

io-uring-test/src/tests/net.rs

@@ -1662,10 +1662,12 @@ pub fn test_udp_sendzc_with_dest<S: squeue::EntryMarker, C: cqueue::EntryMarker>
                 // First SendZc notification
                 11 => {
                     assert!(cqueue::more(cqe.flags()));
+                    assert!(!cqueue::notif(cqe.flags()));
                 }
                 // Last SendZc notification
                 0 => {
                     assert!(!cqueue::more(cqe.flags()));
+                    assert!(cqueue::notif(cqe.flags()));
                 }
                 _ => panic!("wrong result for notification"),
             },

src/cqueue.rs

@@ -333,3 +333,11 @@ pub fn more(flags: u32) -> bool {
 pub fn sock_nonempty(flags: u32) -> bool {
     flags & sys::IORING_CQE_F_SOCK_NONEMPTY != 0
 }
+
+/// Returns whether this completion event is a notification.
+///
+/// This corresponds to the `IORING_CQE_F_NOTIF` flag,
+/// currently used by the [SendZc](crate::opcode::SendZc) operation.
+pub fn notif(flags: u32) -> bool {
+    flags & sys::IORING_CQE_F_NOTIF != 0
+}

src/opcode.rs

@@ -1662,6 +1662,12 @@ opcode! {
     ///
     /// A fixed (pre-mapped) buffer can optionally be used from pre-mapped buffers that have been
     /// previously registered with [`Submitter::register_buffers`](crate::Submitter::register_buffers).
+    ///
+    /// This operation might result in two completion queue entries.
+    /// See the `IORING_OP_SEND_ZC` section at [io_uring_enter][] for the exact semantics.
+    /// Notifications posted by this operation can be checked with [notif](crate::cqueue::notif).
+    ///
+    /// [io_uring_enter]: https://man7.org/linux/man-pages/man2/io_uring_enter.2.html
     pub struct SendZc {
         fd: { impl sealed::UseFixed },
         buf: { *const u8 },