Linux中的clock_gettime

Question

MAIN.c :

int sum()
{
    int x = 100, y =200, z;
    z =x;
    z=y;
    printf("value is %d", z);
    return 1;

}

int main()
{
    double starttime, stoptime;
    int a=1, b=10, c;
    starttime = GetTimeStamp();          // I am calculating time here.
    printf("start  time is %f", starttime);
    c =a;
    printf("value is %d", c);
    c =b;
    printf("value is %d", c);
    c= a+b;

        printf("value is %d", c);
        printf("value is %d", c);
        printf("value is %d", c);
        printf("value is %d", c);    

    stoptime =GetTimeStamp();    // I want to know the time here.
    printf("stop  time is %f", stoptime);
    return 0;

}

TIMER.c :

#define BILLION  1000000L
typedef unsigned int uint32_t;

int GetTimeStamp()
{
struct timespec start;

//double startTime;

           // if( (startTime = clock_gettime( CLOCK_REALTIME, &start)) == -1 )
            if((clock_gettime( CLOCK_REALTIME, &start)) == -1 )
            {
              perror("clock gettime");

            }
         //  startTime =start.tv_sec + 0.0000001 * start.tv_nsec;  // to make it milli 


        return ((start.tv_sec + 0.0000001 * start.tv_nsec)/(double)BILLION);
}

timer.h :

#ifndef TIMESTAMP_H_
#define TIMESTAMP_H_
int GetTimeStamp();

#endif /* TIMESTAMP_H_ */

I want to create a free running timer and want to get the time when it start executing and also the time when it finish the execution. So I created a timer as above in TIMER.c and I am using that in main.c. I am calling the GetTimeStamp() in MAIN.c at the begging of the program and again at the end of time. The output : it is showing the same time for both starttime and stop time as 1594.0000000

Answer 1

Your multiplier for tv_nsec is wrong, and your constant BILLION is wrong too. The prefix n (nano) means 10^-9, and a billion is equal to 10^9. Change:

    return ((start.tv_sec + 0.0000001 * start.tv_nsec)/(double)BILLION);

to:

    return 1e9 * start.tv_sec + start.tv_nsec;        // return ns time stamp

or:

    return 1e6 * start.tv_sec + start.tv_nsec * 1e-3; // return µs time stamp

or:

    return 1e3 * start.tv_sec + start.tv_nsec * 1e-6; // return ms time stamp

or:

    return start.tv_sec + start.tv_nsec * 1e-9;       // return seconds time stamp

depending on what units you want to use for your timestamp.

As already pointed out elsewhere, you might want to use eg a double for your timestamp, rather than an int , since this gives you better range and resolution and it's also what you're already using in main .

double GetTimeStamp(void)
{
    ...

    return 1e9 * start.tv_sec + start.tv_nsec;        // return ns time stamp
}