Execute a task cyclic using posix threads

Question

I'm searching for a solution to start a method every 20ms. I found some solutions for c++ but am still searching for a solution in c which uses posix threads. I dont want to execute the method and wait for n milliseconds as this will take 20ms + X. I really want to start every 20 ms.. does someone has any idea?

Answer 1

The clock_nanosleep() POSIX function has an absolute deadline mode:

#define NSEC_PER_SEC 1000000000
/* Initial delay, 7.5ms */
const long start_delay_ns = 7500000;
/* Cycle time, 20ms */
const long cycle_time_ns = 20000000;
struct timespec deadline;

clock_gettime(CLOCK_MONOTONIC, &deadline);
deadline.tv_nsec += start_delay_ns;
deadline.tv_sec += deadline.tv_nsec / NSEC_PER_SEC;
deadline.tv_nsec %= NSEC_PER_SEC;

for (;;)
{
    struct timespec now;

    /* Sleep until deadline */
    while (clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &deadline, NULL) != 0)
        if (errno != EINTR)
            return; /* error handling here */

    cyclic_function(arguments);    /* Your cyclic function */

    /* Calculate next deadline */
    deadline.tv_nsec += cycle_time_ns;
    deadline.tv_sec += deadline.tv_nsec / NSEC_PER_SEC;
    deadline.tv_nsec %= NSEC_PER_SEC;

    clock_gettime(CLOCK_MONOTONIC, &now);
    if (now.tv_sec > deadline.tv_sec || (now.tv_sec == deadline.tv_sec && deadline.tv_nsec > now.tv_nsec))
        return; /* time overrun error handling here */
}