#include <stdio.h>
#include <sys/mman.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>

#define DATA_SIZE 5*1024*1024
#define MAX_LEN 1*1024*1024
#define LOOP_TIMES 100
int main(){
   unsigned int len,ans;
   char *str1,*src1;
   str1 = (char *)mmap(NULL, DATA_SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

   struct timespec tv0,tv;
   for(len=1; len<=MAX_LEN; len*=2) {
      memset(str1, 'a', DATA_SIZE);
      src1 = str1;  // +offset
      src1[len] = '\0';

      clock_gettime(CLOCK_REALTIME, &tv0);
      for(int k=0; k<LOOP_TIMES; k++){
         ans = strlen(src1);
      }
      clock_gettime(CLOCK_REALTIME, &tv);
      tv.tv_sec -= tv0.tv_sec;
      if ((tv.tv_nsec -= tv0.tv_nsec) < 0) {
              tv.tv_nsec += 1000000000;
              tv.tv_sec--;
      }
      printf("length: %u  time: %ld.%.9ld\n",ans, (long)tv.tv_sec, (long)tv.tv_nsec);

      if( ans != len)
         printf("ERROR! len is %u,ans is %u\n",len,ans); //verify length
   }
   munmap(str1,DATA_SIZE);
   return 0;
}