cmidi cmidi - 17 days ago 6
Linux Question

sem_timedwait with CLOCK_MONOTONIC_RAW/CLOCK_MONOTONIC

The example code for

int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
uses
CLOCK_REALTIME
as the time source from
clock_gettime(struct timespec *timeout)
but this is susceptible to system clock time changes for example some other process changing time backwards.

Is there a support for
sem_timedwait
to support
CLOCK_MONOTONIC
time source

below is some example code for reference.

struct timespec ts;
sem_t sema;
sem_init(&sema, 0, 0)
int ret;
if ( -1 != (ret = clock_gettime(CLOCK_REALTIME, &ts))){
ts.tv_sec += 1;
return sem_timedwait(sema, &ts);
}

Answer

Is there a support for sem_timedwait to support CLOCK_MONOTONIC time source

Short answer: no.

But you could implement one if you're not using a 3rd party library or C++11 and don't need cross-platform compatibility:

#include <cstring> // memset
#include <ctime>   // DEV_NOTE: some systems might need -lrt
#include <csignal> // DEV_NOTE: csignal contains a reference to CLOCK_MONOTONIC
#include <semaphore.h>
#if !defined(CLOCK_MONOTONIC)
    #error CLOCK_MONOTONIC is not defined
#endif

typedef struct timespec tick_t;

static tick_t monotonic_tick()
{
    tick_t tmp;
    if (clock_gettime(CLOCK_MONOTONIC, &tmp) != 0) {
        std::memset(&tmp, 0, sizeof(tick_t));
        // error, throw std::exception(std::strerror(errno))
    }
    return tmp;
}

static double elapsed_us(tick_t init, tick_t end)
{
    return ((end.tv_sec - init.tv_sec) * 1000000) + (static_cast<double>((end.tv_nsec - init.tv_nsec)) / 1000);
}

static double elapsed_ms(tick_t init)
{
    return (elapsed_us(init, monotonic_tick()) / 1000);
}

static int sem_timed_wait(sem_t& sem, unsigned long timeout_ms)
{
    if (timeout_ms == 0) {
        if (sem_trywait(&sem) == 0) {
            return 0;
        }
    } else {
        tick_t start = monotonic_tick();
        do {
            if (sem_trywait(&sem) == 0) {
                return 0;
            }
        } while (elapsed_ms(start) <= timeout_ms);
    }
    return -1;
}

Then to use it:

#include <iostream>
#include <pthread.h>

void* thread_fn(void* val)
{
    sem_t* sem = static_cast<sem_t*>(val);
    std::cout << std::endl << pthread_self() << " thread started" << std::endl;

    if (sem_timed_wait(*sem, 1000) == 0) {
        std::cout << std::endl << pthread_self() << " got it, sleeping 2 seconds..." << std::endl;
        sleep(2); // sleep 2 seconds
        std::cout << pthread_self() << " returning..." << std::endl;
        // don't forget to release since we acquired the lock
        sem_post(sem);
    } else {
        std::cout << pthread_self() << " timeout" << std::endl;
    }
    std::cout << pthread_self() << " thread returning" << std::endl;
    return NULL;
}

int main(int argc, char* argv[])
{
    sem_t sem;
    pthread_t t1, t2;
    sem_init(&sem, 0, 1); // binary semaphore

    std::cout << "Creating..." << std::endl;
    pthread_create(&t1, NULL, thread_fn, static_cast<void*>(&sem));
    pthread_create(&t2, NULL, thread_fn, static_cast<void*>(&sem));

    std::cout << "Joining..." << std::endl;
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    std::cout << "Leaving..." << std::endl;
    return 0;
}

The above works on a wide array of *nix systems to include the BSD line. If you need a cross platform way of doing this, Windows and Apple have simpler mechanisms to do this.

Hope that can help.