Ci stavo pensando ed ero curioso di sapere come si codificasse una sveglia a ripetizione efficiente in C? Vuoi impostare un orario di sveglia e quindi compensare il tempo con l'equivalente tempo ms di un giorno (o due giorni o una settimana a seconda di quanto spesso è necessario ripetere)? E poi sondare periodicamente per vedere se i tempi sono uguali? Sembra una soluzione molto inefficiente, almeno per me. Ero interessato a come una sveglia avrebbe funzionato in modo programmatico.
Ci sono molti modi per ottenere questo risultato, tutto a seconda del tipo di ambiente con cui stai lavorando. Un'altra preoccupazione è ovviamente la latenza che ti puoi permettere.
Se stai lavorando su un sistema operativo moderno, molto probabilmente hai un'API che metterà il thread in attesa fino a quando non sarà arrivato il momento. Posix ha nanosleep() e C11 ha la funzione thread_sleep() per fare questo.
Se utilizzi sistemi operativi più primitivi o programmati direttamente per l'hardware incorporato, potresti dover ricorrere ad attendere interruzioni periodiche, o anche usare loop occupati.
Aggiornamento: per gestire più allarmi utilizzando l'approccio thread_sleep() , potresti ad esempio memorizzare tutti gli avvisi in arrivo in un elenco ordinato. Dormi finché non scatta il primo allarme, fai quello che devi fare e poi calcola il tempo fino al prossimo allarme e torna a dormire. Questo approccio utilizza solo un thread del timer, che sarà molto efficiente purché soddisfi i requisiti di latenza.
per un timer ripetuto.
Usa setitimer()
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NAME
getitimer, setitimer - get or set value of an interval timer
SYNOPSIS
#include <sys/time.h>
int getitimer(int which, struct itimerval *curr_value);
int setitimer(int which, const struct itimerval *new_value,
struct itimerval *old_value);
DESCRIPTION
The system provides each process with three interval timers, each
decrementing in a distinct time domain. *When any timer expires, a sig‐
nal is sent to the process*, and the timer (potentially) restarts.
ITIMER_REAL decrements in real time, and delivers SIGALRM upon expi‐
ration.
ITIMER_VIRTUAL decrements only when the process is executing, and
delivers SIGVTALRM upon expiration.
ITIMER_PROF decrements both when the process executes and when the
system is executing on behalf of the process. Coupled
with ITIMER_VIRTUAL, this timer is usually used to pro‐
file the time spent by the application in user and ker‐
nel space. SIGPROF is delivered upon expiration.
Timer values are defined by the following structures:
struct itimerval {
struct timeval it_interval; /* next value */
struct timeval it_value; /* current value */
};
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
The function getitimer() fills the structure pointed to by curr_value
with the current setting for the timer specified by which (one of
ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF). The element it_value is
set to the amount of time remaining on the timer, or zero if the timer
is disabled. Similarly, it_interval is set to the reset value.
The function setitimer() sets the specified timer to the value in
new_value. If old_value is non-NULL, the old value of the timer is
stored there.
Timers decrement from it_value to zero, generate a signal, and reset to
it_interval. A timer which is set to zero (it_value is zero or the
timer expires and it_interval is zero) stops.
Both tv_sec and tv_usec are significant in determining the duration of
a timer.
Timers will never expire before the requested time, but may expire some
(short) time afterward, which depends on the system timer resolution
and on the system load; see time(7). (But see BUGS below.) Upon expi‐
ration, a signal will be generated and the timer reset. If the timer
expires while the process is active (always true for ITIMER_VIRTUAL)
the signal will be delivered immediately when generated. Otherwise the
delivery will be offset by a small time dependent on the system load‐
ing.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
EFAULT new_value, old_value, or curr_value is not valid a pointer.
EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF;
or (since Linux 2.6.22) one of the tv_usec fields in the struc‐
ture pointed to by new_value contains a value outside the range
0 to 999999.
CONFORMING TO
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
etc.) instead.
NOTES
A child created via fork(2) does not inherit its parent's interval
timers. Interval timers are preserved across an execve(2).
POSIX.1 leaves the interaction between setitimer() and the three inter‐
faces alarm(2), sleep(3), and usleep(3) unspecified.
The standards are silent on the meaning of the call:
setitimer(which, NULL, &old_value);
Many systems (Solaris, the BSDs, and perhaps others) treat this as
equivalent to:
getitimer(which, &old_value);
In Linux, this is treated as being equivalent to a call in which the
new_value fields are zero; that is, the timer is disabled. Don't use
this Linux misfeature: it is nonportable and unnecessary.
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