POSIX timer and clock

The CLOCK_REALTIME clock is a systemwide clock, visible to all processes running on the system and it is defined in time.h header file.

The CLOCK_REALTIME clock measures the amount of time that has elapsed since 00:00:00 January 1, 1970 Greenwich Mean Time (GMT). 

Clock Functions are below

Function	Description
clock_getres	Returns the resolution of the specified clock
clock_gettime	Returns the current value for the specified clock
clock_settime	Sets the specified clock to the specified value

#include 
#include 

main()
{
struct timespec ts;

      /* Call time */
printf("time returns %d seconds\n", time(NULL);)
       /* Call clock_gettime */

clock_gettime(CLOCK_REALTIME, &ts);
printf("clock_gettime returns:\n");
printf("%d seconds and %ld nanoseconds\n", ts.tv_sec, ts.tv_nsec);
}

The timespec data structure consists of two members, tv_sec and tv_nsec, and takes the following form:

typedef struct timespec
  { time_t tv_sec; /* Seconds */
     long tv_nsec; /* Nanoseconds */
 } timespec_t;

struct itimerspec {
       struct timespec it_interval;   /* Timer interval     */
       struct timespec it_value;      /* Initial expiration */
};

Member	Zero	Non-Zero
it_value	No expiration value Disarm the timer	Expiration value Arm the timer
it_interval	No reload value Use as a one-shot timer	Interval reload value Use as a periodic timer

specify a timer that executes only once, 5.25 seconds from now

mytimer.it_value.tv_sec = 5; 

mytimer.it_value.tv_nsec = 250000000; 

mytimer.it_interval.tv_sec = 0; 

mytimer.it_interval.tv_nsec = 0;  

timer to execute 15 seconds from now and then at 0.5 second intervals

mytimer.it_value.tv_sec = 15;

mytimer.it_value.tv_nsec = 0; 

mytimer.it_interval.tv_sec = 0; 

mytimer.it_interval.tv_nsec = 500000000;  

Timer Functions are below.

Function	Description
timer_create	Returns a unique timer ID used in subsequent calls to identify a timer based on the systemwide clock
timer_delete	Removes a previously allocated, specified timer
timer_getoverrun	Returns the timer expiration overrun count for the specified timer
timer_gettime	Returns the amount of time before the specified timer is due to expire and the repetition value
timer_settime	Sets the value of the specified timer either to an offset from the current clock setting or to an absolute value