nanosleep() wait for at least 15 ms? - c

Question

In order to make a good timing in an emulator, I have to wait for around 50 microseconds.
When I use nanosleep, for example with a duration of 10 microsecs, I measure a delay of 15 millisec ! I use mingw on windows with the following example code :
#include <stddef.h>
#include <time.h>
#include <stdio.h>
int main()
{
struct timespec startx, endx;
struct timespec req={0};
req.tv_sec=0;
req.tv_nsec=10000; // 10_000 ns = 10 microseconds
for (int i=0; i<10; i++)
{
clock_gettime(CLOCK_MONOTONIC, &startx);
nanosleep(&req, NULL);
clock_gettime(CLOCK_MONOTONIC, &endx);
if (endx.tv_sec-startx.tv_sec)
printf("%li microsecs\n",((endx.tv_nsec-startx.tv_nsec)/1000)+1000000);
else
printf("%li microsecs\n",(endx.tv_nsec-startx.tv_nsec)/1000);
}
return 0 ;
}
And the result is :
12528 microsecs
19495 microsecs
14890 microsecs
14229 microsecs
14657 microsecs
14824 microsecs
14724 microsecs
21074 microsecs
13697 microsecs
13893 microsecs
I guess I'm wrong somewhere....
If someone has an idea. I could also avoid the problem with a "do nop while (end-start) < 50usec"...
Thanks.

Answer 1

When I run the posted code through a compiler (gcc) the compiler outputs the following:
gcc -ggdb3 -Wall -Wextra -Wconversion -pedantic -std=gnu11 -c "untitled1.c" -o "untitled1.o"
untitled1.c: In function ‘main’:
untitled1.c:16:57: error: ‘err’ undeclared (first use in this function)
16 | if (nanosleep(&req, NULL)) printf("err : %i\n", err);
| ^~~
untitled1.c:16:57: note: each undeclared identifier is reported only once for each function it appears in
untitled1.c:18:18: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘__syscall_slong_t’ {aka ‘long int’} [-Wformat=]
18 | printf("%d microsecs %d\n",(endx.tv_nsec-startx.tv_nsec)/1000, endx.tv_sec-startx.tv_sec);
| ~^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| int __syscall_slong_t {aka long int}
| %ld
untitled1.c:18:31: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘__time_t’ {aka ‘long int’} [-Wformat=]
18 | printf("%d microsecs %d\n",(endx.tv_nsec-startx.tv_nsec)/1000, endx.tv_sec-startx.tv_sec);
| ~^ ~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| int __time_t {aka long int}
| %ld
Compilation failed.
When compiling, always enable the warnings, then fix those warnings.
to start, suggest casting the result of the call to time() to be a long int
as to your question:
context switches take time AND the cpu(s) are probably busy with other tasks AND the task scheduler only looks at the list of tasks ready to run periodically , so your request will not be immediately serviced.
to obtain a more accurate timing, do not depend on the OS. Rather use one of the hardware timers to produce a interrupt event when you want the elapsed time to expire. Then use an interrupt handler to handle the interrupt event from the hardware timer.