timers(2)

NAME

timers: timer_create(), timer_delete(), timer_getoverrun(), timer_gettime(), timer_settime() — timer operations

SYNOPSIS

#include <time.h> 

int timer_create( 
     clockid_t clock_id, 
     struct sigevent *evp, 
     timer_t *timerid 
); 

int timer_delete( 
     timer_t timerid 
); 

int timer_getoverrun( 
     timer_t timerid 
); 

int timer_gettime( 
     timer_t timerid, 
     struct itimerspec *value 
); 

int timer_settime( 
     timer_t timerid, 
     int flags, 
     const struct itimerspec *value, 
     struct itimerspec *ovalue 
); 

DESCRIPTION

timer_create()

The timer_create() function creates a per-process timer using the specified clock, clock_id, as the timing base. The timer_create() function returns, in the location referenced by timerid, a timer ID of type timer_t used to identify the timer in timer requests. This timer ID will be unique within the calling process until the timer is deleted. The particular clock, clock_id, is defined in <time.h>. The timer whose ID is returned will be in a disarmed state upon return from timer_create().

The evp argument, if non-NULL, points to a sigevent structure. If the sigev_notify member of evp is SIGEV_SIGNAL, then the structure should also specify the signal number to be sent to the process on timer expiration. The signal to be sent is specified in the sigev_signo field of evp. If the sigev_notify member of evp is SIGEV_NONE, no notification is sent. If evp is NULL, then a default signal is sent to the process. The defaults for the clocks CLOCK_REALTIME, CLOCK_VIRTUAL, and CLOCK_PROFILE are SIGALRM, SIGVTALRM, and SIGPROF.

Per-process timers are not inherited by a child process across a fork() and are disarmed and deleted by an exec().

timer_delete()

The timer_delete() function deletes the specified timer, timerid, previously created by the timer_create() function. If the timer is armed when timer_delete() is called, the behavior is as if the timer is automatically disarmed before removal. Any pending notifications from the timer remain.

timer_settime()

The timer_settime() function sets the time until the next expiration of the timer specified by timerid from the it_value member of the value argument and arms the timer if the it_value member of value is non-zero. If the specified timer was already armed when timer_settime() is called, this call resets the time until next expiration to the value specified. If the it_value member of value is zero, the timer is disarmed. Any pending notifications from the timer remain.

If the flag TIMER_ABSTIME is not set in the argument flags, timer_settime() behaves as if the time until next expiration is set equal to the interval specified by the it_value member of value. That is, the timer will expire in it_value nanoseconds from when the call is made.

If the flag TIMER_ABSTIME is set in the argument flags, timer_settime() behaves as if the time until next expiration is set equal to the difference between the absolute time specified by the it_value member of value and the current value of the clock associated with timerid. That is, the timer will expire when the clock reaches the value specified by the it_value member of value. If the specified time has already passed, the function will succeed and the expiration notification is made.

The reload value of the timer is set to the value specified by the it_interval member of value. When a timer is armed with a non-zero it_interval, a periodic (or repetitive) timer is specified.

Time values that are between two consecutive non-negative integer multiples of the resolution of the specified timer are rounded up to the larger multiple of the resolution. A quantization error will not cause the timer to expire earlier than the rounded-up time value.

If the argument ovalue is not NULL, the function timer_settime() stores, in the location referenced by ovalue, a value representing the previous amount of time before the timer would have expired or zero if the timer was disarmed, together with the previous timer reload value. The members of ovalue are subject to the resolution of the timer, and are the same values that would be returned by a timer_gettime() call at that point in time.

timer_gettime()

The timer_gettime() function stores the amount of time until the specified timer, timerid, expires and the timer's reload value into the space pointed to by the value argument. The it_value member of this structure will contain the amount of time before the timer expires, or zero if the timer is disarmed. This value is returned as the interval until timer expiration, even if the timer was armed with absolute time. The it_interval member of value will contain the reload value last set by timer_settime().

timer_getoverrun()

Only a single signal is delivered to the process for a given timer at any point in time. When a timer for which a signal is still pending expires, no signal is delivered, and a timer overrun has occurred. When a timer expiration signal is delivered to a process, the timer_getoverrun() function returns the timer expiration count for the specified timer. The overrun count returned contains the number of extra timer expirations which occurred between the time the signal was generated and when it was delivered, up to but not including an implementation defined maximum of DELAYTIMER_MAX. If the number of such extra expirations is greater than or equal to DELAYTIMER_MAX, then the overrun count is set to DELAYTIMER_MAX. The value returned by timer_getoverrun() applies to the most recent expiration signal delivery for the timer. If no expiration signal has been delivered for the timer, the meaning of the overrun count returned is undefined.

RETURN VALUE

Upon successful completion, timer_create() returns zero and updates the location referenced by timerid to a timer_t which can be passed to the per-process timer calls. Otherwise, timer_create() returns -1 and sets errno to indicate the error. The value of timerid is undefined if an error occurs.

Upon successful completion, timer_delete() returns zero. Otherwise, timer_delete() returns -1 and sets errno to indicate the error.

Upon successful completion, timer_settime() returns zero and updates the location referenced by ovalue, if ovalue is non-NULL.

Upon successful completion, timer_gettime() returns zero and updates the location referenced by value, if ovalue is non-NULL. Otherwise, timer_gettime() returns -1 and sets errno to indicate the error.

Upon successful completion, timer_getoverrun() returns the timer expiration overrun count as explained above. Otherwise, timer_getoverrun() returns -1 and sets errno to indicate the error.

ERRORS

If any of the following conditions occur, the timer_create() function returns -1 and sets errno (see errno(2)) to the corresponding value:

EAGAIN: The system lacks sufficient signal queuing resources to honor the request.
EAGAIN: The calling process has already created all of the timers it is allowed by this implementation.
EINVAL: The specified clock ID is not defined.
EFAULT: The timerid or evp argument points to an invalid address.
ENOSYS: The function timer_create() is not supported by this implementation.

If any of the following conditions occur, the timer_delete() function returns -1 and sets errno to the corresponding value:

EINVAL: The timer ID specified by timerid is not a valid timer ID.
ENOSYS: The function timer_delete() is not supported by this implementation.

If any of the following conditions occur, the timer_settime(), timer_gettime(), and timer_getoverrun() functions return -1 and set errno to the corresponding value:

EINVAL: The timerid argument does not correspond to an ID returned by timer_create(), but not yet deleted by timer_delete().
EINVAL: The value structure passed to timer_settime() specified a nanosecond value less than zero or greater than or equal to 1000 million.
EFAULT: The value or ovalue argument points to an invalid address.
ENOSYS: The timer_settime(), timer_gettime(), and timer_getoverrun() functions are not supported by this implementation.

EXAMPLES

Create a timer, set it to go off in one minute, and deliver a SIGUSR1 signal:

#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <signal.h> 
#include <time.h> 

timer_t timerid; 
struct itimerspec one_minute = {0} ; 

void handler() 
{ 
     int overrun = timer_getoverrun(timerid); 

     if (overrun == -1) { 
          perror("handler: timer_getoverrun()"); 
          exit(1); 
     } 
     (void)printf("Timer expired, overrun count was %d\n", 
               overrun); 
} 

int main() 
{ 
     struct sigaction sigact; 
     struct sigevent sigev; 

     one_minute.it_interval.tv_sec = 0; 
     one_minute.it_interval.tv_nsec = 0; 
     one_minute.it_value.tv_sec = 60; 
     one_minute.it_value.tv_nsec = 0; 

     sigact.sa_handler = (void (*)(int))handler; 
     sigemptyset(&sigact.sa_mask); 
     sigact.sa_flags = 0; 

     if (sigaction(SIGUSR1, &sigact, (struct sigaction *)NULL) 
          == -1) { 
          perror("sigaction"); 
          exit(1); 
     } 

     sigev.sigev_notify = SIGEV_SIGNAL; 
     sigev.sigev_signo = SIGUSR1; 

     if (timer_create(CLOCK_REALTIME, &sigev, &timerid) 
          == -1) { 
          perror("timer_create"); 
          exit(1); 
     } 

     if (timer_settime(timerid, 0, &one_minute,  
          (struct itimerspec *)NULL) == -1) { 
          perror("timer_create"); 
          exit(1); 
     } 

     pause(); 
     if (timer_delete(timerid) == -1) { 
          perror("timer_delete"); 
          exit(1); 
     } 
     return 0; 
} 

AUTHOR

timer_create(), timer_delete(), timer_settime(), timer_gettime(), and timer_getoverrun() were derived from the proposed IEEE POSIX P1003.4 standard, draft 14.

STANDARDS CONFORMANCE

timer_create(): POSIX.4

timer_delete(): POSIX.4

timer_getoverrun(): POSIX.4

timer_gettime(): POSIX.4

timer_settime(): POSIX.4