clock_gettime()

Get the current time of a clock

Synopsis:

#include <time.h>

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

Arguments:

clock_id
The ID of the clock whose time you want to get; one of:
  • CLOCK_REALTIME — the standard POSIX-defined clock. Timers based on this clock wake up the processor if it's in a power-saving mode.
  • CLOCK_SOFTTIME — this clock is active only when the processor isn't in a power-saving mode. For example, an application using a CLOCK_SOFTTIME timer to sleep wouldn't wake up the processor when the application was due to wake up. This will allow the processor to enter a power-saving mode.

    While the processor isn't in a power-saving mode, CLOCK_SOFTTIME behaves the same as CLOCK_REALTIME.

  • CLOCK_MONOTONIC — this clock always increases at a constant rate and can't be adjusted.
  • A clock ID returned by clock_getcpuclockid() or pthread_getcpuclockid() , representing the amount of time the process or thread has spent actually running.
tp
A pointer to a timespec structure where clock_gettime() can store the time. This function sets the members as follows:
  • tv_sec — the number of seconds since 1970.
  • tv_nsec — the number of nanoseconds expired in the current second. This value increases by some multiple of nanoseconds, based on the system clock's resolution.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The clock_gettime() function gets the current time of the clock specified by clock_id, and puts it into the buffer pointed to by tp.

Returns:

0
Success.
-1
An error occurred (errno is set).

Errors:

EFAULT
A fault occurred trying to access the buffers provided.
EINVAL
Invalid clock_id.
ESRCH
The process associated with this request doesn't exist.

Examples:

/*
 * This program calculates the time required to
 * execute the program specified as its first argument.
 * The time is printed in seconds, on standard out.
 */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>

#define BILLION  1000000000L;

int main( int argc, char** argv )
  {
    struct timespec start, stop;
    double accum;

    if( clock_gettime( CLOCK_REALTIME, &start) == -1 ) {
      perror( "clock gettime" );
      return EXIT_FAILURE;
    }

    system( argv[1] );

    if( clock_gettime( CLOCK_REALTIME, &stop) == -1 ) {
      perror( "clock gettime" );
      return EXIT_FAILURE;
    }

    accum = ( stop.tv_sec - start.tv_sec )
             + (double)( stop.tv_nsec - start.tv_nsec )
               / (double)BILLION;
    printf( "%lf\n", accum );
    return EXIT_SUCCESS;
  }

Classification:

POSIX 1003.1 TMR

Safety:  
Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes

Last modified: 2013-12-23

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