I need better solution to get time of execution program in C

I am using this

 clock_t start = clock();

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 printf("Time elapsed: %f\n", ((double)clock() - start) / CLOCKS_PER_SEC);

I would like to get time in nanoseconds.

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If you're on a Unix-like system you can most likely use the clock_gettime() function. In Linux, it is found in -lrt, most other systems I've tried have it in the system C library.

It should be noted that x86 has an rdtsc instruction that is extremely precise.

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Use the struct timespec as follows to obtain the time of execution of the code in nanoseconds precision

struct timespec start, end;
clock_gettime(CLOCK_REALTIME,&start);
/* Do something */
clock_gettime(CLOCK_REALTIME,&end);

It returns a value as ((((unsigned64)start.tv_sec) * ((unsigned64)(1000000000L))) + ((unsigned64)(start.tv_nsec))))

Hope this answer will be more helpful for you to get your desired execution time in nanoseconds.