What are the different values of rusage structure in getrusage() system call argument?

Question

We use getrusage() system call to find different values of resources it takes two arguments in which the first argument is RUSAGE_SELF or RUSAGE_CHILDREN, the other argument is a structure named rusage. This structure has many elements which can be accessed and give values but what does all of these elements represent?

#include <sys/resource.h>
#include <sys/time.h>
#include <unistd.h>

void print_cpu_time()
{
 struct rusage usage;
 getrusage (RUSAGE_SELF, &usage);
 printf ("CPU time: %ld.%06ld sec user, %ld.%06ld sec system\n",
     usage.ru_utime.tv_sec, usage.ru_utime.tv_usec,
     usage.ru_stime.tv_sec, usage.ru_stime.tv_usec);
}

int main()
{
    print_cpu_time();
} 

This program shows values of user time and system time.

What do the other elements of the structure represent and how can they be used in real-life programs, like I am getting value 0 for all other elements of structure if I am trying to access them. So how can I use them to get a value other than 0?

EDIT : I have written a program to find the value of ru_inblock and ru_oublock. It is giving the output as 0 for ru_inblock and 8 for ru_oublock for any input given. Why is this so? The code is as follow

#include <stdio.h>
#include <sys/resource.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

// a struct to read and write 
struct person 
{ 
  int id; 
  char fname[20]; 
  char lname[20]; 
}; 

int main () 
{ 
  FILE *outfile; 
  char ch;
  struct person Stu;
int r;

  outfile = fopen ("student.dat", "w"); 
  if (outfile == NULL) 
  { 
    fprintf(stderr, "\nError opened file\n"); 
    exit (1); 
  } 
   do
              {
                      printf("\nEnter Roll : ");
                      scanf("%d",&Stu.id);
                      scanf("%*c");
                      printf("Enter First Name : ");
                      scanf("%s",Stu.fname);
                      scanf("%*c");
                      printf("Enter Last Name : ");
                      scanf("%s",Stu.lname);

                      fwrite(&Stu,sizeof(Stu),1,outfile);

                      printf("\nDo you want to add another data (y/n) : ");
                      scanf("%*c");
                      ch = getchar();


              }
            while(ch=='y' || ch == 'Y');



  if(fwrite != 0) 
    printf("contents to file written successfully !\n"); 
  else
    printf("error writing file !\n"); 
 fclose (outfile); 

 outfile = fopen ("student.dat", "r"); 
  if (outfile == NULL) 
  { 
    fprintf(stderr, "\nError opened file\n"); 
    exit (1); 
  } 

  struct person input; 

  while(fread(&input, sizeof(struct person), 1, outfile)) 
        printf ("id = %d name = %s %s\n", input.id, 
        input.fname, input.lname); 


 fclose (outfile);

  struct rusage r_usage;

  r=getrusage(RUSAGE_SELF,&r_usage);
  printf("\n%d\n",r);
  printf("Memory usage = %ld\n",r_usage.ru_maxrss);
  printf("\ninput operations : %ld \n", r_usage.ru_inblock);
  printf("\noutput operations : %ld \n", r_usage.ru_oublock);


  return 0; 
}