From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Thu, 4 Aug 2011 22:36:12 +0000 (-0400)
Subject: Add ring buffer comment about shm
X-Git-Tag: v1.9.1~311
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=2432c3c9bdce3c94632f7c619165ab1278a69551;p=lttng-ust.git

Add ring buffer comment about shm

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index 01d0f5e8..b45ec543 100644
--- a/libringbuffer/ring_buffer_frontend.c
+++ b/libringbuffer/ring_buffer_frontend.c
@@ -55,6 +55,22 @@
 #define max(a, b)	((a) > (b) ? (a) : (b))
 #endif
 
+/*
+ * Use POSIX SHM: shm_open(3) and shm_unlink(3).
+ * close(2) to close the fd returned by shm_open.
+ * shm_unlink releases the shared memory object name.
+ * ftruncate(2) sets the size of the memory object.
+ * mmap/munmap maps the shared memory obj to a virtual address in the
+ * calling proceess (should be done both in libust and consumer).
+ * See shm_overview(7) for details.
+ * Pass file descriptor returned by shm_open(3) to ltt-sessiond through
+ * a UNIX socket.
+ *
+ * Since we don't need to access the object using its name, we can
+ * immediately shm_unlink(3) it, and only keep the handle with its file
+ * descriptor.
+ */
+
 /*
  * Internal structure representing offsets to use at a sub-buffer switch.
  */