From: Pekka Paalanen <pekka.paala...@collabora.co.uk>
Explain carefully why we need two roundtrips, not just one, not just
dispatch and roundtrip, but two roundtrips after creating the
wl_registry object.
Signed-off-by: Pekka Paalanen <pekka.paala...@collabora.co.uk>
---
clients/simple-shm.c | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/clients/simple-shm.c b/clients/simple-shm.c
index c1cb386..996a8a9 100644
--- a/clients/simple-shm.c
+++ b/clients/simple-shm.c
@@ -388,6 +388,36 @@ create_display(void)
wl_display_roundtrip(display->display);
+ /*
+ * Why do we need two roundtrips here?
+ *
+ * wl_display_get_registry() sends a request to the server, to which
+ * the server replies by emitting the wl_registry.global events.
+ * The first wl_display_roundtrip() sends wl_display.sync. The server
+ * first processes the wl_display.get_registry which includes sending
+ * the global events, and then processes the sync. Therefore when the
+ * sync (roundtrip) returns, we are guaranteed to have received and
+ * processed all the global events.
+ *
+ * While we are inside the first wl_display_roundtrip(), incoming
+ * events are dispatched, which causes registry_handle_global() to
+ * be called for each global. One of these globals is wl_shm.
+ * registry_handle_global() sends wl_registry.bind request for the
+ * wl_shm global. However, wl_registry.bind request is sent only after
+ * the first wl_display.sync, so the reply to the sync comes before
+ * the initial events of the wl_shm object.
+ *
+ * When the reply to the first sync comes, the server may or may not
+ * have sent initial wl_shm events. Therefore we need the second
+ * wl_display_roundtrip() call here.
+ *
+ * The server processes the wl_registry.bind for wl_shm first, and
+ * the second wl_display.sync next. During our second call to
+ * wl_display_roundtrip() the initial wl_shm events are received and
+ * processed. Finally, when the reply to the second wl_display.sync
+ * arrives, it guarantees we have processed all wl_shm initial events.
+ */
+
if (!(display->formats & (1 << WL_SHM_FORMAT_XRGB8888))) {
fprintf(stderr, "WL_SHM_FORMAT_XRGB32 not available\n");
exit(1);
--
2.0.4
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