Demo of how mmap() affects VIRT, RES, SHR memory usage in Linux

mmap-memory-usage.c

// Demo of how mmap() affects VIRT, RES, SHR memory usage in Linux.
//
// A useful resource for more background:
// https://techtalk.intersec.com/2013/07/memory-part-2-understanding-process-memory/

// Required for:
// * `O_TMPFILE`
// * `fallocate()`
#define _GNU_SOURCE

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>


int main(int argc, char const *argv[])
{
  if (argc != 2) {
    fprintf(stderr, "Usage:\n%s MAP_SHARED\n%s MAP_PRIVATE\n", argv[0], argv[0]);
    exit(1);
  }

  int map_flag =
    (strcmp(argv[1], "MAP_SHARED") == 0) ? MAP_SHARED :
    (strcmp(argv[1], "MAP_PRIVATE") == 0) ? MAP_PRIVATE :
    0;

  printf("Using %s (numeric value %d)\n", argv[1], map_flag);

  // Anonymous unnamed file that does not show in the file system:
  //int fd = open("tmpdir", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);
  // To observe file contents in the file system:
  int fd = open("tmpdir/mappedfile", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
  if (fd == -1) {
    perror("open tmpdir");
    fprintf(stderr, "Please `mkdir tmpdir/` first.\n");
    exit(1);
  }

  const size_t length = 1024 * 1024 * 1024;

  if (fallocate(fd, FALLOC_FL_ZERO_RANGE, 0, length) == -1) {
    perror("fallocate");
    exit(1);
  }

  // map_flag (see `man 2 mmap`):
  // * MAP_SHARED: Changes file contents on disk.
  // * MAP_PRIVATE: Does NOT change file contents on disk; uses copy-on-write.
  char * region = mmap(NULL, length, PROT_READ | PROT_WRITE, map_flag, fd, 0);

  if (region == MAP_FAILED) {
    perror("mmap");
    exit(1);
  }

  puts("You should now see 1 GB VIRT in htop. Press Enter to mutate mapped bytes...");
  getchar();

  // Mutate all bytes in the mmapped region.
  for (size_t i = 0; i < length; i++) {
    region[i] += 1;
  }

  // Tell the user what they should expect to see.
  if (map_flag == MAP_SHARED) {
    puts("Because of MAP_SHARED: You should now see 1 GB VIRT, RES and SHR each in htop.");
    // You should now also see tmpdir/mappedfile contents changed,
    // unless you used `O_TMPFILE` in which case the file is invisible.
  } else if (map_flag == MAP_PRIVATE) {
    puts("Because of MAP_PRIVATE: You should now see 1 GB VIRT and RES each in htop. Press Enter to terminate...");
    // You should NOT see any change in tmpdir/mappedfile.
  } else {
    puts("Impossible.");
  }
  puts("Press Enter to terminate...");
  getchar();

  return 0;
}