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Editorial Team
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Asked: 2026-06-02T15:26:38+00:00

I am trying to use v4l2_buffer’s timestamp value (type timeval) to synchronize images captured

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I am trying to use v4l2_buffer’s timestamp value (type timeval) to synchronize images captured from a UVC webcam to external events.

However the timestamp is not the same as the system time, or the up time, etc:

printf("image captured at %ld, %ld\n",
   buffer->timestamp.tv_sec, 
   buffer->timestamp.tv_usec);

struct timeval tv;
gettimeofday(&tv, 0);
printf("current time %ld, %ld\n", tv.tv_sec, tv.tv_usec);

Results in

image captured at 367746, 476270
current time 1335083395, 11225

My uptime is 10 days.

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1 Answer

  1. Editorial Team

    According to http://comments.gmane.org/gmane.linux.drivers.video-input-infrastructure/39892 some v4l2 drivers (including the UVC one) do not use the realtime clock (wall time) but rather a monotonic clock that counts from a not specified point in time. On Linux, this is the boot time (i.e. uptime), however (and I suspect this is the cause of your mismatch) only the time that the computer was actually running (i.e. this clock does not run when the computer is suspended).

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