diff --git a/docs/configuration/frame-outputs.md b/docs/configuration/frame-outputs.md
index 784eb39..68e7a97 100644
--- a/docs/configuration/frame-outputs.md
+++ b/docs/configuration/frame-outputs.md
@@ -40,7 +40,8 @@ Frame geometry is deliberately not a key. An ImageStreamIO stream's geometry is
 lifetime, so the writer recreates the stream whenever the geometry changes from what is allocated.
 
 Unlike the FITS writer, this output posts a semaphore per frame, so it is the right attachment point
-for anything that has to wake up on every frame.
+for anything that has to wake up on every frame. The semaphores live inside the segment, so a reader
+opens it read-write; segments are created `0666` so any local user can attach.
 
 ### Readers
 
diff --git a/utils/shared_memory_writer.cpp b/utils/shared_memory_writer.cpp
index 6bb30eb..f3c8f80 100644
--- a/utils/shared_memory_writer.cpp
+++ b/utils/shared_memory_writer.cpp
@@ -16,7 +16,7 @@
 namespace {
 
   constexpr int NUM_KEYWORDS = 3;         // FRAMENO, TIMESTMP, SEQNUM
-  constexpr mode_t SEGMENT_MODE = 0664;   // group-writable so a co-user can clean up a leftover
+  constexpr mode_t SEGMENT_MODE = 0666;   // readers open read-write to wait on the in-segment semaphores
 
   void set_long_keyword(IMAGE_KEYWORD &keyword, const char *name, int64_t value,
                         const char *comment) {