/* Measure Linux overhead from mmap().
 *
 * It doesn’t appear to depend at all on the size of the region being
 * mapped; it always takes about 4–6 μs per mmap call on my N3700
 * laptop, up to a limit of some 65515 mmap calls (a rather suggestive
 * number).
 */
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) {
  int n = atoi(argv[1]), m = atoi(argv[2]);
  char *filename = "nk-of-zeroes.tmp";
  int fd = open(filename, O_WRONLY | O_CREAT, 0666);
  if (fd < 0) {
    perror(filename);
    return 1;
  }

  char zeroes[n];
  memset(zeroes, 0, n);
  if (write(fd, zeroes, sizeof(zeroes)) < 0) {
    perror("write");
    return 1;
  }
  close(fd);

  fd = open(filename, O_RDONLY);
  if (fd < 0) {
    perror("read open");
    return 1;
  }

  for (int i = 0; i < m; i++) {
    if (MAP_FAILED == mmap(0, n, PROT_READ, MAP_SHARED, fd, 0)) {
      perror("mmap");
      return 1;
    }
  }

  close(fd);
  unlink(filename);

  return 0;
}