[PATCH:libXi] Expand comment on the memory vs. reply ordering in XIGetSelectedEvents()

Alan Coopersmith alan.coopersmith at oracle.com
Mon Apr 29 18:39:34 PDT 2013

Unpacking from the wire involves un-interleaving the structs & masks,
which wasn't obvious to me the first time I read it, so make notes
before I forget again.

Signed-off-by: Alan Coopersmith <alan.coopersmith at oracle.com>

Feel free to reword or clarify to improve this, just figured I'd save the
next person the 15 minutes of head scratching I went through until I realized
the comment describes the non-interleaved memory layout generated later in the 
function, even though it appears right before the interleaved reply parsing.

 src/XISelEv.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/XISelEv.c b/src/XISelEv.c
index fa7eb54..f871222 100644
--- a/src/XISelEv.c
+++ b/src/XISelEv.c
@@ -135,8 +135,14 @@ XIGetSelectedEvents(Display* dpy, Window win, int *num_masks_return)
     _XRead(dpy, (char*)mask_in, reply.length * 4);
-    /* Memory layout of the XIEventMask for a 3 mask reply:
-     * [struct a][struct b][struct c][masks a][masks b][masks c]
+    /*
+     * This function takes interleaved xXIEventMask structs & masks off
+     * the wire, such as this 3 mask reply:
+     *   [struct a][masks a][struct b][masks b][struct c][masks c]
+     * And generates a memory buffer to be returned to callers in which
+     * they are not interleaved, so that callers can treat the returned
+     * pointer as a simple array of XIEventMask structs, such as:
+     *   [struct a][struct b][struct c][masks a][masks b][masks c]
     len = reply.num_masks * sizeof(XIEventMask);

More information about the xorg-devel mailing list