Linux C++ userspace app real time sleep function (POSIX, Raspberry Pi) - c++

Question

I need a function which suspends my program in µs, it should be real time, so if I call it with 50µs the thread should stop for exactly 50µs.
My C++ program is running on a Raspberry Pi with normal Raspbian installed.
I wrote this example program which uses the posix time functions to suspend an measure the suspend time.
#include <cstdlib>
#include "stdint-gcc.h"
#include "signal.h"
#include <time.h>
#include <cerrno>
#include <cstdio>
#include <iostream>
#include <cstring>
#define S_IN_NS 1000000000UL
#define MS_IN_NS 1000000UL
#define US_IN_NS 1000UL
#define GET_TIME_CLOCK CLOCK_MONOTONIC
using namespace std;
int main(int argc, char** argv) {
struct timespec newTimeStamp;
struct timespec oldTimeStamp;
struct timespec sleeptime;
sleeptime.tv_sec = 0;
sleeptime.tv_nsec = 50000; //50us
if (clock_gettime(GET_TIME_CLOCK, &oldTimeStamp) == -1)
cout << "Could not get clock time! ERRNO: " << strerror(errno);
if ((clock_nanosleep(CLOCK_MONOTONIC, 0, &sleeptime, NULL)) == -1)
cout << "Sleep failed! ERRNO: " << strerror(errno);
if (clock_gettime(GET_TIME_CLOCK, &newTimeStamp) == -1)
cout << "Could not get clock time! ERRNO: " << strerror(errno);
uint64_t measuredSec = (newTimeStamp.tv_sec - oldTimeStamp.tv_sec);
int32_t measuredNs = (newTimeStamp.tv_nsec - oldTimeStamp.tv_nsec);
uint64_t diffus = (((measuredSec * S_IN_NS) + measuredNs + 500) / 1000UL);
uint64_t diffns = (((measuredSec * S_IN_NS) + measuredNs));
cout << "Diffns:" << diffns << " Diffus:" << diffus << endl;
return 0;
}
Build commands:
arm-bcm2708hardfp-linux-gnueabi-g++ -lrt -c -g -MMD -MP -MF "build/Debug/GNU_ARM_HARDFP-Linux-x86/main.o.d" -o build/Debug/GNU_ARM_HARDFP-Linux-x86/main.o main.cpp
arm-bcm2708hardfp-linux-gnueabi-g++ -lrt -o dist/Debug/GNU_ARM_HARDFP-Linux-x86/timetest build/Debug/GNU_ARM_HARDFP-Linux-x86/main.o
Result (chrt - manipulate real-time attributes of a process):
pi#raspberrypi ~ $ sudo chrt 99 ./timetest
Diffns:130994 Diffus:131
pi#raspberrypi ~ $ sudo chrt 99 ./timetest
Diffns:135994 Diffus:136
pi#raspberrypi ~ $ sudo chrt 99 ./timetest
Diffns:138993 Diffus:139
The program should sleep for exactly 50us but I measured 130-139us.
If I change the GET_TIME_CLOCK define to CLOCK_PROCESS_CPUTIME_ID the cpu time (excluding the sleeptime) is measured (So as I understand it).
Result:
pi#raspberrypi ~ $ sudo chrt 99 ./timetest
Diffns:89000 Diffus:89
pi#raspberrypi ~ $ sudo chrt 99 ./timetest
Diffns:86000 Diffus:86
pi#raspberrypi ~ $ sudo chrt 99 ./timetest
Diffns:88000 Diffus:88
It takes around 80-90µs to do the clock_nanosleep() function, even if I change the sleeptime to 500µs!
So is there any way to suspend a thread for exactly an amount of time (µs) in a C++ userspace application on Raspbian?
thx

Answer 1

If you need to sleep for such a precise amount of time, you probably need to use a spin loop with a check of the current time. This will consume rather a lot of power (and generate heat), but it's a fairly reliable and portable way to do it. Another idea is to try the ideas on this page: http://blog.regehr.org/archives/794