lavu: add av_gettime_relative()

These functions are using the POSIX clock_gettime() function with the
CLOCK_MONOTONIC clock id. If these are not present on the targeted
platform, the new functions will fallback on using the original realtime functions
av_gettime() and av_usleep().

Monotonic support can be added on other platforms with their
equivalent native system API eventually if possible.

Whenever time is requested to measure relative time, the monotonic clock,
when available, is superior to the system realtime clock because it is
not affected by discontinuous jumps in the system time

In a future step, offering the flexibility to let the user choose between
rt and monotonic clock for avdevices packets will be investigated.

It is very easy to experience the issues that this patch attempt to address
by rewinding back in the past the system time while ffmpeg is running.

this is breaking the ffmpeg report printing (ffmepg.c:print_report()) and
the the rate emulator functionality (-re) without the patch.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

1 files changed, 15 insertions, 0 deletions

diff --git a/libavutil/time.h b/libavutil/time.h
index 90eb436949..910d28e85f 100644
--- a/libavutil/time.h
+++ b/libavutil/time.h
@@ -29,6 +29,21 @@
 int64_t av_gettime(void);
 
 /**
+ * Get the current time in microseconds since some unspecified starting point.
+ * On platforms that support it, the time comes from a monotonic clock
+ * This property makes this time source ideal for measuring relative time.
+ * If a monotonic clock is not available on the targeted platform, the
+ * implementation fallsback on using av_gettime().
+ */
+int64_t av_gettime_relative(void);
+
+/**
+ * Indicates with a boolean result if the av_gettime_relative() time source
+ * is monotonic.
+ */
+int av_gettime_relative_is_monotonic(void);
+
+/**
  * Sleep for a period of time.  Although the duration is expressed in
  * microseconds, the actual delay may be rounded to the precision of the
  * system timer.