Obsolete Timer APIs#

From The Epoch: `alarm()`#

Only one timer per process
- Repeated calls to alarm() cancel a previously set alarm timer
One-second granularity
Expiry notification: SIGALRM

code/alarm.cpp#

#include <signal.h>
#include <unistd.h>
#include <assert.h>
#include <print>

static void timer_expired(int sig)
{
    assert(sig == SIGALRM);
    const char msg[] = "expired\n";
    write(STDOUT_FILENO, msg, sizeof(msg));
}


int main()
{
    struct sigaction sa = { 0 };
    sa.sa_handler = timer_expired;

    int rv = sigaction(SIGALRM, &sa, nullptr);
    if (rv == -1) {
        perror("sigaction");
        return 1;
    }

    unsigned int prev_remain = alarm(1);
    std::println("previous remaining: {}", prev_remain);
    rv = pause();
    if (rv == -1) {
        if (errno == EINTR)
            std::println("pause() interrupted");
        else {
            perror("pause");
            return 1;
        }
    }
    return 0;
}

Evolution: Interval Timers#

Not only one, but three per-process timers available
Drawbacks
- Definition predates suspend/resume
- Not immune against time sets (e.g. NTP)
- Conflict with alarm() timers

`ITIMER_REAL`	Wall clock time; `SIGALRM` at expiration
`ITIMER_VIRTUAL`	Profiling: counts user-mode CPU time; `SIGVTALRM` at expiration
`ITIMER_PROF`	Profiling: counts total CPU time (user and system); `SIGPROF` at expiration

All in all: very limited still