From 8c88e071c5cefa1559989d0292cdfba6b438793c Mon Sep 17 00:00:00 2001
From: Keith Packard <keithp@keithp.com>
Date: Thu, 30 Jan 2014 12:25:04 -0800
Subject: Fencing using pipes. Not used in practice

Signed-off-by: Keith Packard <keithp@keithp.com>
---
 pipefence.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 pipefence.c

diff --git a/pipefence.c b/pipefence.c
new file mode 100644
index 0000000..a62517b
--- /dev/null
+++ b/pipefence.c
@@ -0,0 +1,112 @@
+/*
+ * Copyright © 2013 Keith Packard
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that copyright
+ * notice and this permission notice appear in supporting documentation, and
+ * that the name of the copyright holders not be used in advertising or
+ * publicity pertaining to distribution of the software without specific,
+ * written prior permission.  The copyright holders make no representations
+ * about the suitability of this software for any purpose.  It is provided "as
+ * is" without express or implied warranty.
+ *
+ * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
+ * OF THIS SOFTWARE.
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+#include <sys/eventfd.h>
+#include <poll.h>
+
+int
+event_fence_create(void)
+{
+	return eventfd(0, EFD_CLOEXEC|EFD_NONBLOCK);
+}
+
+int
+event_fence_await(int fd)
+{
+	struct pollfd fds = { .fd = fd, .events = POLLIN };
+	int	ret;
+
+	for (;;) {
+		ret = poll (&fds, 1, -1);
+		if (ret < 0)
+			return -errno;
+		if (ret > 0 && fds.revents & POLLIN)
+			return 1;
+	}
+}
+
+int
+event_fence_test(int fd)
+{
+	struct pollfd fds = { .fd = fd, .events = POLLIN };
+	int	ret;
+
+	ret = poll (&fds, 1, 0);
+	if (ret < 0)
+		return -errno;
+	if (ret > 0 && fds.revents & POLLIN)
+		return 1;
+	return 0;
+}
+
+int
+event_fence_trigger(int fd)
+{
+	uint64_t	v = 1;
+	return write (fd, &v, sizeof (v));
+}
+
+int
+event_fence_reset(int fd)
+{
+	int		ret;
+	uint64_t	v;
+
+	ret = read(fd, &v, sizeof (v));
+	if (ret == -1 && errno == EAGAIN)
+		ret = 0;
+	return ret;
+}
+
+int main(int argc, char **argv) {
+	int	fence = event_fence_create();
+	int	i;
+
+	switch (fork()) {
+	case 0:
+		for (;;) {
+			printf ("await fence\n");
+			if (event_fence_await(fence) < 0) {
+				printf ("waiter done\n");
+				break;
+			}
+			printf ("reset fence\n");
+			event_fence_reset(fence);
+		}
+		break;
+	case -1:
+		break;
+	default:
+		for (i = 0; i < 10; i++) {
+			sleep(1);
+			printf ("trigger fence\n");
+			event_fence_trigger(fence);
+		}
+		printf ("trigger done\n");
+		break;
+	}
+	return 0;
+}
-- 
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