Problem in measuring time!!

Hannes Mayer h.mayer at
Sat Oct 9 18:49:33 CEST 2004

Tarik KHOUTAIF wrote:
> I’m using the kernel RTAI 2.4.20 _ RTHAL5 to develop a protocol of 
> control/command. I work under  PERIODIC mode (and not ONESHOT mode). I 
> have two real time tasks the first "Slave" who await a message on the 
> serial port COM2, and the second "Master" which sends the message on the 
> COM1.
> Once the Slave task receives the message, returns a ACK to the Master. 
> To measure time in the loop of asservissement of my system, and the time 
> of execution of certain functions, I initialised start_rt_timer(1E1) in 
> the nitialisation of the serials port.
> The time of the transmission RTIME Time_transmission Time_send_command = 
> rt_get_time_ns(); //in Master task Time_receive_command = 
> rt_get_time_ns(); //in Slave task Time_send_response  = 
> rt_get_time_ns(); Time_receive_response = rt_get_time_ns(); 
> Time_transmission = time_receive_command – Time_send_command;
> The problem is that :
> Time_send_command = Time_send_response and Time_receive_command = 
> Time_receive_response!! Is what you can inform me or to show me how to 
> program differently to solve this problem???


I don't know if you need nanosecond resolution, but when I must measure
time (in kernel modules; in microseconds) I use this:

time = tv.tv_sec+(1.0*tv.tv_usec)/1000000;

That may be not the preferred way in RT context, but it seems to work.

Best regards,

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