clock_gettime

Name

clock_gettime, clock_settime, clock_getres, clock_getcpuclockid
- clock and time functions

Library

libc.lib

Synopsis

#include <sys/time.h>

int clock_gettime (clockid_t clock_id, struct timespec *tp);

int clock_settime (clockid_t clock_id, const struct timespec *tp);

int clock_getres (clockid_t clock_id, struct timespec *tp);

int clock_getcpuclockid (pid_t pid, clockid_t* clock_id);

Return values

All the above APIs return 0 on success and -1 on failure.

Detailed description

The clock_gettime and clock_settime allow the calling process to retrieve or set the value used by a clock which is specified by clock_id.

The clock_id argument can be one of four values: CLOCK_REALTIME for time that increments as a wall clock should, CLOCK_MONOTONIC which increments in SI seconds, CLOCK_VIRTUAL for time that increments only when the CPU is running in user mode on behalf of the calling process, or CLOCK_PROF for time that increments when the CPU is running in user or kernel mode. As of now Symbian OS exposes only wall clock time at user level,hence we support only CLOCK_REALTIME for all the clock-based APIs.

The structure pointed to by tp is defined in

#include <sys/time.h>as:

struct timespec {
        time_t  tv_sec;         /* seconds */
        long    tv_nsec;        /* and nanoseconds */
};

The resolution (granularity) of a clock is returned by the clock_getres system call. This value is placed in a (non-NULL) *tp.

The clock ID of the CPU-time clock of the process specified by pid is returned by the clock_getcpuclockid system call. If pid is zero,the clock ID of the CPU-time clock of the process making the call, is returned in. *clock_id.

Examples

#include <time.h>
#include <stdio.h>
int clock_user()
{
        struct timespec tp;
        int retval;
        clockid_t clockid;
        clock_getres (CLOCK_REALTIME, &tp); //Call clock_getres
        printf ("Real time-clock resolution is %d seconds and %d nanoseconds\n", tp.tv_sec, tp.tv_nsec);
        clock_getcpuclockid (0 ,&clockid); //Call clock_getcpuclockid with pid = 0
        printf ("The clock id for the current process is %d\n", clockid);
        tp.tv_sec = 0;
        tp.tv_nsec = 100;
        retval = clock_settime (CLOCK_REALTIME, &tp); //Call clock_settime with 100ns
        printf ("clock_settime returned %d\n", retval);
        clock_gettime (CLOCK_REALTIME, &tp); //Call clock_gettime to fill tp
        printf ("Time from real time-clock is %d seconds and %d nanoseconds\n", tp.tv_sec, tp.tv_nsec);
        return 0;
}

Output

Real time-clock resolution is 0 seconds and 1000000 nanoseconds
The clock id for the current process is 0
clock_settime returned 0
Time from real time-clock is 0 seconds and 70663000 nanoseconds

Errors

The following error codes may be set in errno:

[`EINVAL`]
	The clock_id argument was not a valid value.
[`EFAULT`]
	The *tp argument address referenced invalid memory.

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