getrusage-t.c strange getrusage() & /proc/PID/status behavio

// avp 2015 strange getrusage() behavior
//   (from http://ru.stackoverflow.com/questions/85287 compare /usr/bin/time & /proc/[PID]/status RSS)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <errno.h>
#include <limits.h>
#include <fcntl.h>

void
usage (int rc)
{
  puts("Usage: ./a.out [-f] [-s] [-r ...] [-?] [-h]\n"
       "\t-?,-h\t this text and exit\n"
       "\t-f\t fork/exec and  wait3() for get resources (as in /usr/bin/time)\n"
       "\t-s\t use system(grep Vm /proc/PID/status) other use FILE *...\n"
       "\t-r\t repeate FILE fgets /proc/PID/status (may be more than one)\n"
       "\t-R\t repeate using open/read /proc/PID/status (may be more than one)"
       );
  exit(rc);
}

int
main (int ac, char *av[])
{
  char cmd[100], str[LINE_MAX * 2];
  struct rusage r;
  pid_t p;
  int i, doit = 1, use_file = 1, grep = 0, repeate = 0;

  for (i = 1; av[i]; i++)
    if (strcmp(av[i], "-f") == 0)
      doit = 0;
    else if (strcmp(av[i], "-s") == 0)
      grep = 1;
    else if (strcmp(av[i], "-r") == 0) {
      use_file = 1;
      repeate++;
    } else if (strcmp(av[i], "-R") == 0) {
      use_file = 0;
      repeate++;
    } else if (strcmp(av[i], "-?") == 0 || strcmp(av[i], "-h") == 0)
      usage(0);
    else {
      printf("unknown option '%s'\n", av[i]);
      usage(1);
    }


  if (doit) {
    int fail = getrusage(RUSAGE_SELF, &r);
    printf("start ru_maxrss: %ld (%s)\n", r.ru_maxrss, fail ? strerror(errno) : "OK");
    if (grep) {
      puts("system grep");
      sprintf(cmd, "grep Vm /proc/%d/status", (int)getpid());
      system(cmd);
    } else {
      puts("file grep");
      sprintf(cmd, "/proc/%d/status", (int)getpid());
      FILE *f = fopen(cmd, "r");
      while (fgets(str, LINE_MAX, f))
    if (strncmp(str, "Vm", 2) == 0)
      fputs(str, stdout);
      fclose(f);
    }
    if (repeate) {
      sprintf(cmd, "/proc/%d/status", (int)getpid());
      printf("... and now repeate with file grep %d times\n", repeate);
      // why we need fopen() for update /proc/[PID]/status and getrusage() ?
      while (repeate--) {
    if (use_file) { // -r -r  -s -f CHANGE /proc/[PID]/status and getrusage() !!!
      FILE *f = fopen(cmd, "r");
      while (fgets(str, LINE_MAX, f))
        if (strncmp(str, "VmRSS", 5) == 0)
          fputs(str, stdout);
      fclose(f);
    } else { // -R -R  -s -f DON'T CHANGE /proc/[PID]/status and getrusage() ???
      int l, fd;
      printf ("read %d ... ", l = read(fd = open(cmd, O_RDONLY), str, LINE_MAX));
      close(fd);
      str[l > 0 ? l : 0] = 0;
      char *t = strstr(str, "VmRSS:");
      if (t) {
        *strchr(t, '\n') = 0;
        puts(t);
      } else
        puts("ERROR /proc");
    }
    fail = getrusage(RUSAGE_SELF, &r);
    printf("mid ru_maxrss: %ld (%s)\n", r.ru_maxrss, fail ? strerror(errno) : "OK");
      }
    }
    fail = getrusage(RUSAGE_SELF, &r);
    printf("fin ru_maxrss: %ld (%s)\n", r.ru_maxrss, fail ? strerror(errno) : "OK");
  } else if (p = fork()) {
    printf("run child %d\n", (int)p);
    p = wait3(0, 0, &r);
    printf("child %d wait3.ru_maxrss: %ld\n", (int)p, r.ru_maxrss);
  } else {
    const char *ar[repeate + 3]; // = {av[0]};
    i = 0;
    ar[i++] = av[0];
    if (grep)
      ar[i++] = "-s";
    while (repeate--)
      ar[i++] = use_file ? "-r" : "-R";
    ar[i] = 0;
    execv(ar[0], ar);
    perror(av[0]);
  }

  return puts("End") == EOF;
}