pselect

$ cat pselect01.c
#include <stdio.h>
#include <fcntl.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>

struct tcase {
	struct timespec tv;
	unsigned int iterations;
};

static unsigned int monotonic_resolution;

static struct tcase tcases[] = {
	{{0, 1000000},  500},
	{{0, 2000000},  500},
	{{0, 10000000}, 300},
	{{0, 100000000},  1},
	{{1, 0},          1},
};

#define MIN(a, b) ((a) < (b) ? (a) : (b))

static struct timespec start_time, stop_time;

static inline struct timespec tst_timespec_diff(struct timespec t1,
                                                struct timespec t2)
{
	struct timespec res;

	res.tv_sec = t1.tv_sec - t2.tv_sec;

	if (t1.tv_nsec < t2.tv_nsec) {
		res.tv_sec--;
		res.tv_nsec = 1000000000 - (t2.tv_nsec - t1.tv_nsec);
	} else {
		res.tv_nsec = t1.tv_nsec - t2.tv_nsec;
	}

	return res;
}

static inline long long tst_timespec_to_us(struct timespec t)
{
	return t.tv_sec * 1000000 + (t.tv_nsec + 500) / 1000;
}

static void verify_pselect(int i)
{
	fd_set readfds;
	struct timespec tv;
	long long requested_us, slept_us = 0;
	int threshold;
	struct tcase *t = &tcases[i];

	FD_ZERO(&readfds);
	FD_SET(0, &readfds);

	tv = t->tv;

	clock_gettime(CLOCK_MONOTONIC, &start_time);
	pselect(0, &readfds, NULL, NULL, &tv, NULL);
	clock_gettime(CLOCK_MONOTONIC, &stop_time);

	printf("expected %lu us, got %llu us\n", tv.tv_nsec/1000, tst_timespec_to_us(tst_timespec_diff(stop_time, start_time)));
}

void main(void)
{
	unsigned int i;
	for (i = 0; i < 4; i++)
		verify_pselect(i);
}