Move to kernel style SPDX license identifiers

[lttng-ust.git] / doc / examples / clock-override / lttng-ust-clock-override-example.c

diff --git a/doc/examples/clock-override/lttng-ust-clock-override-example.c b/doc/examples/clock-override/lttng-ust-clock-override-example.c
index 766e66c9a13a612862aaa23b09112caf6ccbc524..b1c3c3c4515532081b79d9b80d6fa0fb8e7b16c7 100644 (file)
--- a/doc/examples/clock-override/lttng-ust-clock-override-example.c
+++ b/doc/examples/clock-override/lttng-ust-clock-override-example.c
@@ -1,33 +1,37 @@
 /*
- * lttng-clock-override-example.c
+ * SPDX-License-Identifier: MIT
  *
- * Copyright (c) 2014 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
- * SOFTWARE.
+ * Copyright (C) 2014 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  */
 
 #include <stdlib.h>
+#include <stdint.h>
 #include <time.h>
 #include <string.h>
 #include <stdio.h>
 #include <lttng/ust-clock.h>
 
+/*
+ * For sake of example, transform time into a coarse clock (freq: 1KHz).
+ * Note that division can be slow on some architectures. Should be
+ * avoided. Use shift and multiplication instead. e.g.:
+ *
+ * Operation: / 1000000ULL
+ * 1/1000000ULL = .000001
+ * 2^19 < 1000000 < 2^20
+ * Add a 10 bits shift to increase accuracy:
+ * 2^(19+10) = 536870912
+ * x * 1 / 2^(19+10) ~= .000001
+ * 537 * 1 / 2^29 = .00000100024044513702
+ * 537 (multiplication factor) is between 2^9 and 2^10.
+ *
+ * In order not to overflow, first right shift by 10, multiply, and right
+ * shift by 19.
+ */
+#define DIV_CLOCK_SHIFT1       10
+#define DIV_CLOCK_MUL          537
+#define DIV_CLOCK_SHIFT2       19
+
 static
 uint64_t plugin_read64(void)
 {
@@ -39,7 +43,9 @@ uint64_t plugin_read64(void)
         * mechanism: we take the monotonic clock, and transform it into
         * a very coarse clock, which increment only at 1KHz frequency.
         */
-       return ((uint64_t) ts.tv_sec * 1000ULL) + (ts.tv_nsec / 1000000ULL);
+       return ((uint64_t) ts.tv_sec * 1000ULL)
+               + ((DIV_CLOCK_MUL * ((uint64_t) ts.tv_nsec >> DIV_CLOCK_SHIFT1))
+                       >> DIV_CLOCK_SHIFT2);
 }
 
 static